Luận văn thạc sĩ Những Mô Hình Dự Trữ với Dòng Cầu Ngẫu Nhiên

BQ
GIAO
Dl)C
& DAO
HOC
QUOC
GIA
TP. HO
CHi
MINH
TRUONG
HOC
KINH
TE
·
HOANG
NGQC
QUANG
NHU'NG
M0
HiNH
Du'
TRU'
•
' -
vOI nONa
c.Au
NGA.u
NHIEN
,.
CHUYEN NGANH : TOAN

E>IEU
KHIEN
MA
s6
:
s.o2.2o
LO!N
AN
Sl
f{HOA
HOC
f{INHTl
NGUOI HUONG DAN KHOA HQC:
PGS-
PTS:
LEVAN
HOT
- TP.
HO
CHI MINH
1998-
CONG TRiNH
E>A
E>U'QC
HOAN
THANH
TRUONG
E>AI
HOC KINH
TE

THUOC
E>AI
HOC QUOC GIA
. . . . .
THANH
PHO HO
CHI
MINH

NGU'OI
HU'ONG
DAN KHOA HOC:
PGS-
PTS:
LEVAN
HOT

-
PHAN BII;N 1: .

-
PHAN BII;N 2: .
an
se
du<;jc
bao v$ t9i
H(>i
dong cham
an Nha nude hop t9i Truong
E>9i

hoc Kinh
te
vao
hoi

gio

ngay

thang

nam 1998.
MUCLUC
. .
LOI
NOI
£>AU····················································································
Chu'ong
I
? / / A
r-{
?
Md
£>AU-
CAC
KHAI
Cv
BAN
.
-

So
lu'<;5c
lich
sll'
ly
thuye't
quan
ly
dt!
tn1'
.
-
Cac
khai
co
ban
.
-
Cac
loc;ti
chi
phi
.
-
Cach
xay
dtfng
mo
hlnh
.

Chu'ong II
MO
HINH
QUAN
L Y Dl)'
TRU
TRONG
TRUONG
HC)P cAc
£>AI
Lu'C)NG xAc
E>JNH
.
A.
Mo
hlnh
don
gian
xac
dinh
muc
d()
b6
sung
voi
gia
thie't dtf tril'
khong
dtt<;1c
thie'u .

-
d€
I .
-
d€
II .
B. Tru'ongh<;1p
yeu
xua't
khi
thie'u
hang
dt!
tn1'
thl
cho
.
C.
Truong
h<;1p
ma't
yeu
khi
yeu
khong
du'<;jc
dap
ung
.
Chu'ong III

MO
HINH
QUAN
LY
Dl)'
TRU
VOl
YEU
CAU
NGAU
NHIEN
.
A.
Md
.
B.
Mieu
ta
xa'p
xi
<Q,r>
-
Mo
hlnh
tru'ong
h<;1p
yeu
khong
du'QC
dap

ung
ngay
se
cho
.
C.
Mieu
ta
xa'p
xi
<Q,r>-
Mo
hlnh
bi
ma't
nhung
nhu
khong
du'QC
dap
ung
ngay
.
D.
Phan
tich
mo
hlnh
va
thi

d1 1
tinh
toan
.
Chu'ong IV.
A A '
""
?
E>IEU
TON
TAl
VA
Sl)'
ON
E>JNH
CUA
.
1 s
0
d ?
h•
A
. t!
ton
tc;tl
uy
n
at
cua
ng

.
2.
Stj
6n
dinh
.
3.
St! phl.l thu()c
cua
(Q*,
r*)
vao
'A,
TI,
I, C .
·
Trang
1
3
3
3
4
7
9
9
12
13
15
20
23

23
25
30
32
38
38
40
43
*****
Quan ly
Dv
tru
la
m()t va'n d€
c'lln
thie't trong m9i
thanh
phlin kinh
te':
trong san xua't kinh doanh, trong dich
Vl;l
cling nhu' trong qu6c phong

Ly thuye't quan ly
dl,i
tru
ra doi
vao
d§u the'
ky

20 voi cong thuc
Wilson xac dinh kh6i
lu'<;lng
hang h6a
b6
sung cho tung
d<;lt.
Nhung ly
thuye't quan ly
dl,i
tru
voi
cac
lu'<;lng
nhien chi
du'QC
phat
me sau the' chie'n II. Be'n nay cung voi slf phat cua
ky
vi tinh,
quan ly
dl,i
tru
du'<;lc
phat
me
v€ ly thuye't cling nhu' trong ung
dl,ing.
Nhung ly thuye't toan
hQc

hi<$n
nhu' giai tich, quy phi tuye'n,
ly thuye't t6i u'u, ly thuye't t6i u'u toan eve, ly thuye't cac qua trlnh nhien
dtt<;lc
sll'
dl;lng
r()ng rai trong nghien cilu quan ly dl! tru.
Ly thuye't nay ngay cang
du'QC
nhi€u nha toan
hQC
ung
dl;lng
Ion tren
the' gioi
quan tiim nhu' Hadley, Whitin, Kovalurko
Ban van
gom
4 chuong:
Chu'ong
I:
Chung
tOi
lu'<;lc
qua lich
sll'
ly thuye't quan ly
dl,i
tru va
cac

khai
ni<$m
co ban,
phan
tich cac chi phi va each xay
dl,ing
mo
hlnh.
Chu'ong II : Trinh
bay
m()t
s6
mo hlnh quan ly dlf
tru
voi
cac
lu'<;lng
xac dinh.
- Mo hlnh don
gian
( cong thuc Wilson)
-
Mo hlnh voi
yeu
CllU
se
cho khi thie'u hang
dl,i
tru.
- Mo hlnh voi

yeu
eli u
bi
m1t khi thie'u hang
dl,i
tru.
Chu'ong
III:
Xet
mo hlnh voi dong cliu Ia
lu'<;lng
nhien
<Q,r>-
m6 hlnh trong hai tru'ong
h<;lp:
Truong
h<;lp
yeu
cliu se cho va tru'ong
h<;lp
yeu
cliu hi
ma"t
khi thie'u
hl;lt
hang
dl,i
tru.
Chu'ong
IV:

Xet
sv
t5n duy nha't
nghi<$m
cling nhu' tinh
6n
dinh cua
mo
hlnh
xet
d chu'ong III.
Trong 3 chu'ong d'au chung toi t6m nhii'ng ke't qua
v'e
ly thuye't
quan
ly dlf trii' duqc trinh
bay
trong
cac
tai
lil$u
(vi
dv
nhu
trong
sach
chuyen khao
cua
Hadley-Whitin (Analysis
of

Inventory System9
mot
s6
chung minh rieng
cua chung toi.
B6ng
g6p chinh cua chung toi d
chuang
IV. Nhii'ng ke't qua chung toi
trinh
bay
d chuang IV theo ch6 chung toi duqc bie't la hoan toan moi. Nhii'ng
ke't qua
d6 giup chung ta bie't duqc
hi$:
Q =
2A.(A
+
TI11(r)
IC
H(r)
= QIC
TIA.
Khi
nao
c6 nghil$m, khi
nao
thl
c6
nghil$m duy nhfft va cho bie't

nghil$m cua
hi$
tren kha vi
theo
cac
tham
s6
A.,
TI,
I, C.
Nhii'ng ke't qua d6 giup chung ta
ra'"t
nhi'eu khi giai quye't nhii'ng
bai
toan
tht;t'c
te'.
*****
Trang 2
I


,

M()
1),:\U -
C:AC:
1\tiAI
f:(}
.

*****
van
quan ly
dv
tn1
thie't hi Ia m(lt van rna
mQi
thanh ph 'an kinh
te',
mQi
kinh te' quan tam. Dt! tn1 rat
c'an
thie't trong kinh te' qu6c
dan,
trong san xua't cong trong dich vv thu'dng cling nhu' trong
qu6c phong.
C6
ra't nguyen nhan
dfin
de'n t6 chile dt! tn1 nguyen
hang h6a. Tren thvc te' khong th€ khong kinh te' ne'u nhu' m6i khi xuat
yeu
c'au
m(lt hang h6a nao d6 ta moi di kie'm d€ thoa man yeu
c'au
d6.
Ne'u
ta khong c6 hang h6a dt! tru thl khach hang mat thoi gian cho
d<;1i,
doi khi khach hang khong mu6n khong th€ cho. Nhu' chung ta

se mat m(lt ph'an
1<;1i
va uy tin cua ta d6i voi khach hang hi giam di,
anh hu'dng de'n kinh doanh lau dai con nhung nguyen nhan khac nua
cling khong kern ph'an
quan
trQng.
Thi dv nhu' gia nguyen nao d6
thay
d6i dang k€ theo mua, khi gia nguyen d6 tha'p, tat nhien ta c6
1<;1i
ne'u
mua dt! tru cho san xuat cho nhung thoi di€m gia nguyen
len cao. So qua m(lt vai nguyen nhan, ta thay dt! tru nguyen la
c'an
thie't.
Trong cong quan ly dt! tru chung ta phai tra loi hai cau hoi sau:
1,
Thoi di€m h6 sung
dv
tru.
2,
Kh6i
lu<;1ng
hang h6a h6 sung
dv
tru la hao nhieu.
Trong
chu'ong nay chung ta se
c6

ga:ng
tra loi hai cau hoi tren trong
m(lt
s6
tinh hu6ng khac nhau. Nhung th6ng quan
ly
dv
tru
ra't
khac nhau
theo tinh
chat cua chung. Vi dv dt! tru
nu'oc
d h6 chua nu'oc
khong chi phv thu(lc vao y mu6n cua ta, rna chu ye'u con phv thu(lc vao
lu'<;1ng
nu'oc
mu'a
cua khu vvc d6 trong thoi gian d6. d day chung ta se khong xet
nhung
va'n
nhu'
Chung ta se chi
xet
va'n
quan ly
dv
tru trong
tru'ong
h<;1p

ta c6
dQ
tv
do nha't djnh, d€ quye't dinh
v'e
thoi di€m va kh6i
lu'<;1ng
hang h6a (thie't hi)
c'an
h6 sung.
Stl'
dvng cac phu'dng phap toan
hQC
ta se chi ra cac quy quan ly
dv
tru, ta phai mieu ta th6ng quan ly dlj tru ngon
ngu
toan
hQC.
mo
Trang 3
ta m9t
thO'ng
ngon ngu tmin h9c thu'ong
du'cjc
g9i la mo hlnh h6a
toan h9c cua
thO'ng
d6. Nhu'ng the' gioi
th\l'c

khong th€ rnieu ta ngon
ngu toan
h9c rn9t each chinh sach d6i. Vl khi tie'n mo hlnh
h6a
chung ta chi c6 th€
xa'p
xi rna thoi. Ne'u ta rno ta
th€
gioi thvc cang chinh
xac thl rno hlnh toan cang phuc
t(;lp.
Cong phan tich cac mo hlnh phuc
t';lp
nhu' se
g;Jp
nhi"eu
kh6 khan khong
din
thie't. Do d6 tuy theo rnuc d9
yeu
c'au
chung ta phai ddn gian h6a the' gioi thvc.
nghien cliu quan ly dt! tru da xua't
tu
lau. Nhung cong thuc
chinh thuc
du'cjc
xac
l'an
d'au

do Ford Harros rut ra vao narn 1915 thu'ong
du'cjc
g9i la cong thuc ddn gian xac djnh kh6i
lu'cjng
b6 sung hay cong
Wilson. Quy€n sach d'au tien vie't
v"e
va'n
d"e
quan ly
dv
tru
la do rn9t c9ng
tac vien cua
H9c Massachuset F.F Raymond vie't vao narn 1931. Nhu'ng
chi
vao cu6i nhung narn cua The' chi€n II, khi nghien cliu chie'n
lu'cjc
phat tri€n r'arn r9 thl cong nghien cliu quan ly dlf tru rnoi
du'cjc
chu
y.
Quy€n sach cua Whitin vie't narn 1953 la quy€n sach vie't tie'ng Anh
xem
xet
rn9t each
c;Jn
ke
rno hlnh quan ly dt! tru nhien. De'n nay
nghien

CUu
quan ly
d\1'
trfi'
dang phat tri€n va da thu
du'cjc
ra't
nhi"eu
thanh qua
trong ly thuy€t ciing nhu' trong ling
dl;lng.
Cac nha nghien cliu da
sii'
dl;lng
kha
nhi'eu nhung cong
Cl;l
toan h9c
d';li
nhu' giai tich, quy
ho(;lch
phi tuy€n,
quy nhien.
Bay gio chung toi xin
lu'cjc
qua rn9t
sO'
khai cd ban. Tru'oc he't d6
1a
nhung chi phi. Trong dt!

tru
san ph§rn c6 ra't
nhi"eu
chi phi. C6 nhung
chi phi khong thay d6i du ta thay d6i each thuc quan ly. Nhung chi phi
d6 ta
se
khong
d"e
toi. Chung ta chi
d"e
toi nhung chi phi thay d6i khi
ta thay
d6i cung each
di"eu
hanh.
Cl;l
th€ c6 5 chi phi sau:
1.
Chi phi lien quan d€n hang (chi phi ti€p
2.
Chi phi lien quan de'n bao quan san ph§rn (con g9i chi phi htu
kho).
3.
Chi phi lien quan de'n giao hang (chi phi giao hang).
4.
Chi phi xua't khi san ph§rn (hang h6a) dt! trii' bj thie'u
hl;lt.
5.
Chi phi lien quan de'n thu va

xU'
ly
du
(chi phi thong
tin).
1/
Chi
phi
tie'p Tru'oc he't d6 la chi phi cho ngu'oi di tie'p va
phu'dng tie'p (giao thong). Sau de'n chi phi cho nhung van ban gia'y
Trang 4
tO
hang, chi phi gia'y to
bao
van
ban,
h6a
ddn, gio
ch(;ly
may, phi bu'u
Nhung chi phi tren ta chia
lam
hai
lo<;1i:
rn9t
lo<;1i
ph1;1
thu9c
vao
khO'i

ht<;1ng
hang
h6a
(san phffrn) tie'p
va
rn9t
lo(;li
khong
phl;l
thu9c
vao
khO'i
lu'<;1ng
hang
h6a tie'p Nhung chi phi
ph1;1
thu9c
vao
khO'i
lu'<;1ng
hang
h6a
tiSp thu'ong
du'QC
tinh
gQp
VaO
gia hang h6a.
Ta
ky chi phi tiSp

ph1;1
thu9c
vao
khO'i
lu'<;1ng
san phffrn tiSp
Ia C (Q) trong d6 Q la
khO'i
lu<;1ng
san
phfirn tie'p Gia
tie"p
trung
b
'
h ? d . .
""
1"
A
1'
C(Q)
m cua
dn
v1
tlep 1eu a

.
. . Q
Ta
lu'u

y
de"n
tru'ong
h<;1p
gia
tie"p
trung blnh cua rn9t
san
phfirn khong d6i va ky C. Trong tru'ong
h<;1p
d6 C(Q) = C.Q. Khong rna'y
khi xay ra tru'ong
h<;1p
tren, nhu'ng trong
t€
thong thu'ong ta c6 xa'p xi
Nay
ta
xet
chi phi khong
phl;l
thUQC
vao
khO'i
lu'<;1ng
tie"p
D6
Ia
nhung chi phi cho gia'y
tO,

thu' tin, tho(;li
va
ca nhung chi phi
tie"p
quan
ly rna khong
phl;l
thu9c
vao
kich
co
cua
hang
h6a
tie"p
Nhung chi phi d6 xua't rn6i fan ta hang.
Ta
gQi
n6 la chi phi
hang. Gia hang ta ky la A. Nhu' t6ng chi phi cho rn9t l'an
b6
sung Ia
A+
C(Q). Trong chung
nao
d6 ta c6 gia
thie"t
gia hang
trong cac l'an hang
la

khong d6i.
Gia
hang
choN
l'an hang Ia NA.
21
Chi phi hiu kho:
Bao
g'Orn
nhung chi phi thue kho, thue
cac
thie"t
bi
bao
quan
trong rn9t
sO'
tru'ong
h<;1p
n6
g'Orn
ca nhung chi phi lien quan
de"n
hao
h1;1t
rna khong phai
lUc
nao
cling tren gia'y to. Nhung rna't
mat

d6 xua't do
hao
h1;1t
khO'i
lu'<;1ng
do bi rna't
dip,
nhii'ng
h<;1i
do
vO'n
bi
t'On
dQng, d6ng quy
bao
thu€.
Khong phai ta't ca cac
chi phi
lu'u
kho thay d6i
giO'ng
nhau
khi ta
thay
d6i rnuc
dQ
va
each
tru. Ra't kh6 tinh
du'<;jc

ta't ca nhii'ng chi phi d6
rn9t each chinh xac.
Ta
ddn gian
h6a
tinh xa'p xi.
Ta
se
coi chi phi
lu'u
kho ty voi gia
thanh
san
phffrn
lu'u
kho
t<;1i
thoi d6.
sO'
ty
se
ky la
Iva
ta coi n6 khong thay d6i
theo
thoi gian va
gQi
la
sO'
chi phi

lu'u
kho. Ta't
nhien
0 < I <
1.
Nhu' chi phi
lu'u
kho x san phfirn voi
gia thanh
C trong rn9t ddn vi thoi gian (vi
d1;1
1 narn) la ICx. Nhu' ta tha'y chi
phi
lu'u
kho rn9t ddn vi san phfirn con ty voi thoi gian
lu'u
kho.
Ne"u
Trang 5
x(t) la
lu'<Jng
hang h6a lttu kho
t';li
thoi diSm t thl chi phi lttu kho trong khoang
thoi gian (tJ,
tz)
se la:
IC
f x(t)dt
3/

Chi phi giao Khi thlfc giao hang cho khach ta phai thlfc
m{)t
nhung
d{)ng
tac
nhu':
la'y
hang
tu
kho ra, xe'p hang len phu'ong
chuyen chd. Doi khi con phai chd hang de'n cho ngu'oi mua. Ngoai ra con
phai lam
h6a don thanh to an, gia'y bao hanh Nhung chi phi d6 ra't khac
nhau, nhu'ng n6i chung chung khong
phl;l
thu{)c
vao each
chc.m
cac phap
quan ly dlf tru.
Do d6 ta se khong xet de'n.
4/ Chi phi xua't khi
san
phfim dtf trt1
hi
thie'u
Ta xet chi phi lien quan
de'n
khi xua't rna ta khong c6 dlf tru,
th6ng he't hang.

C'an
phan hai tru'ong
h<Jp:
i) Truimg
h<Jp
khdch himg
se
chif: Trong thlfc kh6 rna xac dinh
du'<Jc
nhung khi khach hang phai cho. Nhung
h';!i
d6 la
sv
thie'u
tlnh cua khach hang d6i voi co sd cua ta nghia la trong tu'ong lai khach
hang
c6 thS tlm
de'n
chu hang khac. N6 anh hudng de'n suy nghi cua khach
hang va nhung ngu'oi
quen cua khach hang
v-e
slj ngheo nan cua co sd ta.
Ngoai
rata
con phai tinh
de'n
nhung chi phi
v-e
nhung thong tin rna ta phai

cung
ca'p
cho khach. Trong tru'ong
h<Jp
th6ng
tv
tieu
thl;l
san phfim dlf
tru
thl ta se tinh de'n gia
tri
rna y m6c, cong nhan phai ngung
lam
vl thie'u
nguyen N6i chung gia chi phi khi thie'u viing cua 1 don vi hang h6a va
khach hang phai cho
c6 thS mieu ta
du'<)c
m{)t
ham II(t) khong giam
theo thoi gian cho
t.
Ham II(t) ra't khac nhau cho nhung th6ng quan ly
khac
nhau. Tren thljc
t€
ra't kh6 xac dinh
m{)t
each chinh xac cua

ham
1\
II(t). Nhu'ng ta c6 thS
xa'p
xi II(t) =
TI
+
f1
t
2i)
Truimg
h<Jp
khdch himg khong chif:
Ta xet th6ng yeu c'au khong cho khi thie'u hang
du
tru;
h(;li
nay khong
phlJ
thu{)c
vao thoi gian. Ne'u nhu' kh6i lu'ong cua m6i yeu
c'au
la 1 don vi thl chi phi cho m6i
yeu
c'au
khong
du'<Jc
thoa man la khong d6i.
Ne'u
nhu' yeu

c'au
c6 kh6i lu'ong
IOn
hon 1 don vi thl chi phi d6
phlJ
thu{)c
vao
kh6i
lu<Jng
cua yeu
c'au
va slf
phlJ
thu{)c
d6 kha phuc Nhung dS don gian
ta se gia thie't
dng
chi phi cho 1 don vi hang h6a hi thie'u
hl;lt
luon luon
khong d6i.
Trang 6
5/ Chi
phi
lien
quan
de'n
thu
va
xll'

ly
dii'
DS
si't
dt,mg
cac each thuc phan tich
thO'ng
quan ly
dv
tnT,
ta
c'an
m()t
b()
thu'ong xuyen cung
ca'p
thong tin
tr(;lng
thai cua
thO'ng
va
til
d6
du'a
ra nhii'ng quye't dinh dieu hanh. Nhung chi phi nay n6i chung
ph\1
thu()c vao phu'ong phap hanh
thO'ng.
N6
gf>m

nhung chi phi cho may
m6c dS
xi't
ly thong tin,
b()
thu thong tin dS thu'ong xuyen thong
bao
gia ca san phfim va
dv
doan nhii'ng
yeu
c'au trong thoi gian toi. Nhu'ng
trong
m()t
sO'
tru'ong
hcJp
ta coi chi phi nay khong
ph\1
thu()c vao phu'ong phap
hanh
thO'ng.
Tie'p theo chung ta se
ban
de'n phu'ong phap
chQn
chie'n
lu'cjc
hanh
thO'ng.

Ml;lc
tieu cua xlt ly mo hlnh quan ly
dv
trii' phu'dng phap toan
hQC
la nho n6 ta
chQn
du'cjc
chie'n
lu'cjc
thich
hcjp
dS hanh sao cho
lcji
thu
du'cjc
la cao nha't chi phi
be
nha't. N6i
va:n
dt
Ia
chQn
phu'ong phap
hanh
t6i ttu.
Ta't nhien ta chi c6 thS
chQn
du'cjc
phu'ong an t6i u'u ne'u

lcji
chi phi la
hii'u
h<;1n.
Theo
tieu chufin nay ta
chQn
chie'n
lu'cjc
lam
eve
d<;1i
h6a
lcJi
blnh quan hang
nam
eve tiSu h6a chi phi blnh quan hang
nam.
Ne'u nhu'
lcji
trong khoang thoi gian t la B(t) va chi phi trong
khoang
thoi
giant
Ia
Z(t) thllcJi blnh quan hang
nam
B va chi phi blnh
quan hang
nam

Z
du'cjc
dinh nghia nhu' sau:
B
=lim
B(t) · Z
=lim
B(t)
'
t t
t->+oo
t->+oo
chQn
phu'ong an theo tieu chufin sau thu'ong la thich
hcjp
hon. Ta't
nhien
se
khong hay, ne'u nhu' t6i ttu h6a
lcji
blnh quan hang
nam
eve tiSu h6a chi phi blnh quan hang
nam
de'n nhung phuong an
chie'n
lu'cjc
khac h£n chie'n
lu'cjc
t6i u'u h6a

lcji
chi phi trong
khoang
thoi gian
sa:p
toi. Nhu'ng ra't may
sv
l<;1i
xay ra khong hoan toan
nhu'
y.
Trong ling
dl;lng
thvc te' ca hai tieu chufin ta de'n hoac la
cung m()t phuong an
hay
1a
2 phuong an khac nhau khong dang
kS.
Ph'an con
l<;1i
ta se
lam
r6 xac dinh cac ye'u
tO'
nhien. DS xac
dinh
lcji
blnh quan hang
nam

chi phi blnh quan hang
nam
trong
Trang 7
di'eu nhung ye'u
t6
nhien
anh
hu'dng thvc
den
qua trinh quan ly'
chung
ta se
sli'
dvng cac phu'ong
phap
cua ly thuyet
xac
sua't. Chung ta se
thay the'
l<ji
blnh quan hang
nam
bilng gia
tri
trung blnh hay con
gQi
la
ky
VQng

toan
hQC
cua chung.
*****
Trang 8
II
MiJ
L
Ul)
TVU
11£5V
CAC
f)A.I
XAC
. . . .
*****
Ta
d'au
xet
quan
ly th6ng
dv
tnT
nghien cuu m()t
va1
mo
hlnh don gian, trong d6 cu'ang
d()
yeu
c'au hofm

toan
xac
djnh va khong
d6i
theo thai gian. Ta't
nhien
tren thvc te' khong
bao
gia
dong
yeu
c'au
l(;li
hoan toan xac djnh rna
bao
gia
cling la m()t
d(;li
lu'<Jng
ngiiu nhien. du
nhung mo hlnh
xac
dinh giup ta
lam
quen
voi phu'ong
phap
phan
tich
se

du'QC
sll'
dt;mg
trong nhung
mo
hlnh phuc
t(;lp
hon.
Hon
nua
nhung ke't qua thu
du'QC
tu
nhung mo hlnh tren cho
phep
ta suy
Xet
tinh chfft cua cac
th6ng phuc
t(;lp
ca khi
yeu
c'au la
d(;li
lu'<Jng
ngiiu nhien.
A.
MO HINH DON GIAN XAC DINH MUC DO
BO
SUNG VOl

GIA THIET
nu
TRU' KHONG
Du'dc
THIEU.
Ta
xet
bai toan
quan
ly
d\1'
tru
voi
lo(;li
c6
mQt
kho chinh, trong di'eu
cu'ang
d()
yeu
c'au A
san
ph§:rnldon vi thai gian (vi
d1;1
1 nam),
hoan
toan
xac djnh, khong d6i
theo
thai gian. Va'n

d'e
ra cho ta la di xac djnh thai
b6
sung va kh6i
lu'<Jng
b6
sung ciia m6i
d<Jt.
Ta
gia thie't
them
la thai
gian tie'p
't
(tuc
Ia
khoang thai gian
tu
luc hang de'n luc hang
du'<Jc
vao
kho) khong d6i
va
khong
ph1;1
thu()c
vao
A
va
kh6i

lu'<Jng
hang
Ngoai
rata
cling gia thie't toan
b()
hang m6i l'an
du'<Jc
giao cling m()t luc
va hang
h6a c6
bao
quan
bao
lau
tuy y rna khong bi hu' hong.
Ta
c6
coi th6ng tie'p
tl;lc
ho(;lt
d()ng trong nhu' trong tu'ong lai.
Ta
gia thie't
hang
h6a
du'<Jc
b6
sung
t(;li

cac thai
to=
o < t
1
<

<
tn
<

voi kh6i
lu'<jng
hang
h6a
b6 sung la
Qo, Q1,
, Qn,···
va
thai b6
sung
tj,
hang
t'On
trong kho la
Sj,
i = 0, 1' 2, Ta't
nhien
So
= 0 va
si

0
'Vi
=
1,
2,
Vl hang h6a
dv
tru
khong khi
nao
du'QC
thie'u.
Si
=
Qi-1
+
Si-1
-
'ATi
0.
Ta
gia thie't
h<$
s6
htu kho I 0 < I < 1 khong
thay d6i theo thai gian va gia hang khong thay d6i cho
cac
l'an hang.
Trang 9
Ngoai ra phi tie'p ta

g<)p
vao
gia thanh
san
phfim, voi chi phi
xU'
ly thong
tin ta gia thie't khong
phl,l
thu<)c
vao
each
hanh
th6ng.
Nhu'
tl;li
thoi di€m
tn
::;
t::;
tn+J.
lu'<Jng
hang
h6a
htu kho
se
la:
x(t) =
Qi
+

Si
-'A
(t-
ti)
i = 0,
1,
2, .
Q
Qo
Sz
: :
to
t6ng chi phi de'n thoi di€m t
se
la:
z(t)
=
A(n
+
1)
+ IC. £ x(t)dt
=
A(n
+
1)
+ Ic:i=. t
x(t)dt
+ IC. f
x(t)dt
i=1 I

" IC
=
A(n
+
1)
+
2
(Qj_
1
+
Si_
1
+
SJTi
+ IC f
(Qi_
1
+
Si-
'A(t-
tJ)dt
I
IC " IC "
z(tn)
=
A(q
+ 1) + 2 Q
iti
+ 2 (Si_, + Si
)Ti

(1)
Ta
n6i z(tn) la tdng chi phi de'n thoi
di€m
tn
ung voi phu'ong
an
(to,

,
tn,
Qo,
QJ.

,
Qn).
Ta
c6 ke't sau:
1) Khi ta
c6
djnh
to
<
t1
<

<
tn
thl cho
mQi

phu'ong an
(to,
tt.

,
tn,
Qo,
Qt.···,
Qn)
thoa
man
Si
=
Qi-1
+
Ti
-'ATi
:2::
0
Vi=
1,
2,

, n c6 1 phu'ong
an
sao cho
(to,

,
tn,

,"Qn)
sao cho
'S:
="Qi-1
+-'S"i-1
-'ATi
= 0
Vi
=
1,
2,

, n c6 1 phu'ong
an
(to,
tt.

,


tn,
Qo,

,
Qn)
sao cho:
=
'Qi_1
+ Si-1)-

'ATi=
0
Vi=
1,
2,

, n
Trang 10
Va
t6ng chi phi z(t
0
)
ung
voi phu'dng
an
sau
luon luon
nho
hon
hoi;ic
chi phi z(
tn)
ung
voi phu'dng
an
tru'oc.
2) Khi ta
c6
dinh
tn

thl
cho
mQi
phu'dng an.
/'. /'.
/'.
/'.
/'-
Vi=
1,
2,

, n d'eu
c6
1 phu'ong
an
(to, t
0
,
Qo,

,
Qn)
sao
cho:
/'.
1:'
/':'.
.
Qi-I

=
ti-I)
Vt =
1,
2,

, n
Va
chi phi z(t
0
)
ung
voi
phu'ong
an
sau
nho
hon
hoi;ic
chi
phi
z(tn)
ung
voi phu'dng
an
tru'oc.
Chang minh:
11
U'ng voi
phuong

an
(to.
t1,

,
tm
Qo.
QJ,

,
Qn)
Voi:
Si
=
Qi-1
+
Si-1
-A.Ti
0
Vi=
1,
2,

, n
ta
c6:
IC " IC "
z(t")
=
A(n

+
1)
+
-L
Q
;_
1
t;
+
-:L
(S;_
1
+
S;)T;
2
i=l
2
i=l
Khi d6
hien
nhien:
si
=
'Qi-1
-
Ti
= o
Chi phi z(t
0
)

ung
voi phu'ong
an
sau
ta"t
nhien
se
la:
n
JC
n n
z
(t")
=
A(n
+
1)
+
-:L
(Q
i-l
t;)
2
i=l
11
da
du'qc
chung
minh.
2/

Gia
sti' (to. t,

' t;
Qo,
Q I'

'
Qn)
Ia 1 phu'dng
an
voi:
Vi =
1,
2,

,
n.
n n n
Thl: Q = L Q
i-1
=
LA.
T i =A. L
Ti
i=l
n=l
i=l
Trang
11

La
1
s6khong
d6i
va
theo
bat
thuc
Bunhiacovski
ta c6:
(t.
)
2
(t.1
z
(t.)
= A
(n
+
1)
+
(I
Q
;_,
)
A
(n
+
1)
+ IC' I ( Q ) ' A

(n
+
1)
+
2IC'
L Q
;_,
2
II.
n
II.
dday:
n Q
Q
=
\fi, i =1, , n
va
ta
T"
=
tn-
to
n
n
0 =
t"
<
t"
= 1
.T

<

<
t"
=
nT
0 I
II
Khi
d6
chi ling
voi
phu'dng
:t"'
'{Jo,

,Qn)
se
1a:
Tu
ke't
tren
ta
suy
ra
d€
tlm
phu'dng
an
sao

cho
chi phi
blnh
quan
hang
nam
nho
nhat
ta
chi
c'an
xet
nhfi'ng phu'dng
an
c6
cac
thai
di€m
b6
sung
hang
dl)'
trfi'
each
nhau,
va
khong
thai
gian
gifi'a

hai
d\ft
b6
sung
hang
dl)'
trfi' ta
se
g<;>i
la
chu
ky. D'ong
thai
ta
chi c'an
xet
nhfi'ng phu'dng
an
sao
cho
thai
di€m
b6
sung
hang
thl
kh6i
lu'\fng
hang
t'on

kho
Ia khong.
Chu
ky
ta
se
ky
laT.
Lu'\fng
hang
b6
sung
m6i
l'an
ta
ky la Q thl
hi€n
nhien
Q
='AT.
S6
chu ky
blnh
quan
cua
1
nam
se
la: 1/T =
'A/Q.

Menh tfe
1:
Chi
phi
blnh
quan
hang
nam
t6ng
chi
phi
cua
1
chu
ky
nhan
voi
s6
chu
ky
blnh
quan
m<)t
nam.
Chung
minh:
tn
t
tn+I
\it

Ta
c6:
IC
l
z(l) =
A(n
+
1)+-nQT
+ IC
Q-'A(t
-t
)]dt
2
T n
r
IC
n(t)
= IC
(Q-
A.(t-
t )]dt <
Q.T
T n 2
Trang 12
1
.
z(t)
A
1
.

n + 1 JC Q T
1
.
n
liD =
liD +-

liD-
1-'>+00
t
1-++00
t 2
1-'>+00
t
Hon
nua: t = nT + (t - nT).
l = !!_T +
(t-
nT
)
t t
1 = lim
n
Nen:
Nhttthe':
Dodo:
I -'>
+OO
t
lim

+co
lim
t-+
+oo
t-
nT
t
n
=
t
1
T
= 0
(Vl 0
::;;
t -
nT
<
T)
z = lim
z (
t)
A
IC . Q
(
AQA
+
.Q
J
=

-+
=
+«>
t T 2
=
(A
+
IC
2 A
=
(A
IC
)
+
-2 Q
.T
Trong d6
A/Q
la
s6
chu
ky
trung
blnh
mot
nam.
A+
(IC/2).QT la tdng chi
phi
trong

mot
chu
ky.
m<$nh
d'e
tren
du'Qc
chung minh.
Menh
i!e 2: Chi phi blnh
quan
hang
nam
dt;tt
gi£1
tii
nho nha't.
Khi:
Q*
v oi gia
tri
d6:
Z op =
.J
2
AIC
A
(Cong thuc Wilson)
Chung minh:
Z =

A'A
+
ICQ
.J2AIC
'A
Q 2 Q 2
Trang 13
Va
chi c6 da'u khi:
A A = IC A .Q * <=> Q
2
* = 2 A A
Q * 2 IC
Vi Q 0 nen:
Q*=
{2AT
Xet
thai
diem
hang:
Ta't
cac thai
diem
hang cfing
each
nhau
va khoang each
giii'a
hai thai
diem

nhau
cfing T = Q*l/
Ta
gia
sli'
thoi
diem
t:
tn:::;;
t
:::;;
tn+I•
ta hang
va
ta't
nhien
ta
se
du'QC
lu'<;jng
hang h6a d6
tl;li
thoi
diem
b6
sung
tn+l
+ m = (n + 1 + m)T voi m
0 nao d6.
Hien

nhien: t +
't
= (n + 1 + m)T.
't
= (n +
1)T-
t +
mT
voi (n +
1)T-
t > 0
Va
(n +
l)T
- t
:::;;
T.
a) Truong
h<;ip
't
ga'p 1
s6
nguyen fan T thi
hien
nhien
(n +
1)T-
t = T
=>
t =

nT
= t
0
•
Do
d6
t(;li
nhii'ng
diem
b6 sung ta
ding
hang.
b) Truong
h<;ip
-r/T khong nguyen:
't
= (n +
l)T-
t +
mT.
't
(n+l)T-t
-=
+m
T T
Vi
O<(n+I)T-t<I
T
Nen
m

=['tiT].
khac
't
= (m +
1)T-
(t-nT).
Khi d6:
't
= (m +
1)T-
f=>
e =
(m
+
1)T-
't.
Trang 14
T£:li
thoi diSm
t,
ht<_jng
hang dlf tru trong kho
se
la:
r =
Q-
A.e=
Q-
A[(m +
1)T-

't]
=
A't-
mAT
=
A't-
mQ
't
la 1
d(;li
ht<;5ng
khong d6i.
chung ta se hang r6i thl diSm
hang
trong kho con
1lu'<;5ng
la:
r = mQ trong d6:
't;
m = ['t/T].
Ta
se
xet
1 tieu chu§'n khac.
T£:li
thoi diSm hang
t,
ta con c6 nhfi'ng
hang da
du<;5c

dS
b6 sung
vao
cac thoi diSm
tn+h

tn+m·
ta't ca c6 m
d<;5t,
nen
t6ng
s6
hang da chua
du'<;5c
la mQ.
Do
d6 t6ng
s6
hang t6n
kho va
s6hang
da
chua la: r + mQ =
Ta
di de'n ke't sau:
Chung ta se hang vao cac thoi diSm rna t6ng
s6
hang da chua
va
s6

hang con htu kho la =
A.
't.
B. TRUONG
HOP
YEU CAU XUAT HIEN
KHI
TRIEU HANG
Du' TRU THI CHO:
Trong ph'an A m9i
yeu
c'au xufft
du<_jc
thoa
man
tuc la khong
khi
nao
thie'u hang dlf
trfi'.
Bay gid ta
xet
tru'ong
h<_jp
t6ng quat hon: Cu6i
cung
ffiQi
yeu
c'fiu
xufft

du'QC
dap ung
d'fiy
du.
d day ta se
xet
tru'ong
h<Jp
m<)t
s6
yeu
c'au xufft nhu'ng khong con
hang dlf
trfi'.
Trong tru'ong
h<;5p
d6
yeu
c'au
se
cho
de'n thoi diSm b6 sug hang
moi.
Ta
gia thie't khi bo sung dlf tru, tru'oc he't ta dap ung nhfi'ng nhu c'au dang
cho, sau d6 moi dap ung nhung
yeu
c'au khac.
Ne'u
nhu' cho

dQi
cua
nhung
yeu
c'au khong
du'<_jc
dap ung ngay,
khhong gay
chota
them
chi phi
them
nao
ca. Ta se cho cho nhung
yeu
c'au
xufft
du Ion luc d6 ta b6 sung
s6
hang dung
s6
nhu c'au dang cho
d6. khac ne'u chi phi
cho
qua
IOn
thl ta
se
khong
bao

gio
dS
thie'u hang
dlf
trfi'
ca. Trong nhung tru'ong
h<jp
khac thl chi phi t6i u'u
d(;lt
du'QC
khi d cu6i
m6i
chu ky hang dlf
tru
bi
thie'u.
Trang 15
Ta
se
gia thie't chi phi cho
m<)t
yeu
c'au voi
khO'i
lu'<Jng
la
m<)t
don vi
hang
h6a

cho la
TI
+
f'It,
trong d6 t
Ia
thoi gian
yeu
c'au phai cho. Nhu'
chi phi
g'Om
hai
ph'an:
Ph'an
1:
TI
Ia
chi phi nhu'
nhau
cho
mQi
yeu
c'au voi
khO'i
lu'<Jng
1 don vi
hang
h6a
phai cho.
Ph'an

2:
Tit ty voi thoi gian
yeu
c'au phai cho.
Tu'ong
W'
nhu' d ph'an A ta chi
din
xet
nhung phu'ong
an
c6 cac toi
b6
sung each d'eu nhau 1 khoang thoi
gianT
(chu ky),
lu'<Jng
hang
h6a
b6
sung m6il'an nhu' nhau
Ia
Q
va
trong
cuO'i
m6i
chu ky
khO'i
lu'<Jng

hang h6a
yeu
c'au chua
du'<Jc
dap ung
c6
cling
m<)t
1u'<Jng
la E 0. Nhu' sau khi
dap
ung
mQi
yeu
c'au cho thl
khO'i
lu'<Jng
hang
dl;l'
trii' con
1a
Q-
E.
Lu'<Jng
hang
h6a
d6
ph1;1c
v1;1
ngay tuc cho nhung

yeu
c'au xua't trong khoang
thoi gian T
1
=
11
A.(Q
-
E)
d d'au
m6i
chu ky, con nhung
yeu
c'au xua't
trong khoang thoi gian T
2
= T - T
1
=
EIA.
<J
m6i
cuO'i
ky thl phai cho.
Q-E
ffinh2
Do
d6 chi phi htu kho trong m6i chu ky
Ia:
ICf'(Q-E-At)dt=

IC(Q-E).T
1
=
IC(Q-E)
2
1 2
2A
Chi phi cho trong m6i chu ky la:
n
fT,
1 1 E
2
I1
E +
I1
.b
A tdt =
I1
E + -
I1
=
I1
E + -
I1
-
2 2 A
T6ng chi phi trong m6i chu ky la:
IC 1 n
A+
-(Q-

&)
2
+ Il& +
-TI
&
2
2 1.
2 1.
Vi
sO'
chu ky blnh
quan
trong 1
nam
la
A./Q
nen
chi phi blnh
quan
hang
nam
la:
(
AA.
IC 2 1 1 n 2
z s Q) =
-+-(Q-s)
+-(llA-s+-lls
)
' Q 2Q Q 2

Trang 16
Bie'n d6i cong thuc tren ta du'qc:
z(&,Q)
=
TI+
IC
(&
_
1cg-
I1A-J
2
2
Q I1+
IC
+
, ,
1
[JCITQ+21CIU+
2AA.(IT+IC)-ITzA.z]
2(IT+IC) Q
Nhin vao cong thuc
tren
ta thffy ne'u voi E
c6
dinh: 0
::;;
E <
+OO
thi
z(E,Q) =

+oo
khi Q = +
oo
Q =
0.
Va
ne'u:
0 < Q <
oo
thi z(E,Q) =
+oo
khi E = +
oo.
khac ta thffy
VE
0 ta c6:
z(c,Q)?.j(Q)=
, ,
1
[/CITQ+2ICIU+
2AA.(IC+m-rrzA_z]z
2(IT+/C) Q
,.,-
?
kh"
ICQ
-
TI
A.
Dau

= xay ra 1 c =
=, ,'
n + IC
Khi
fi
> 0 thi nhien f(Q) = +
oo.
Khi Q = +
oo
z(E,Q) nho nhfft khi E =
+oo,
Q =
+oo
thi
fi
= 0.
Va
ham
s6
f(Q) la
ham
s6khong
tang, tuc la:
2AA.IC-
II
2
A-
2
0 <=>
rr

?.
A;c
= 5
Khi d6:
Zmin
=
ITA
Tom
z chi gia tri nho nhfft voi E =
+OO
khi va chi khi:
f:r
= 0, IT
8.
Trong tru'ong hqp: E =
+oo
thi ta khong c'an
he$
th6ng dlf
tn1.
Ta
cu
cho cho kh6i luqng
yeu
c'au du
IOn
(nhu'ng thoi gian cho khong
qua dai c6 anh hu'dng de'n
ban
chfft

cua
he$
th6ng) ta moi
b6
sung m()t
lu'<;Jng
hang dung kh6i lu'qng cua nhii'ng
yeu
c'au d6. Trong nhii'ng tru'ong
h<;Jp
COn
Z chi gia tri nho nhfft
CllC
(E*,
Q*)
0::;;
E*
<
+OO,
0 <
Q*
<
+OO
oz
=
Il+
IC
(£
_
ICQ-

Ill
J = O
O£
Q
TI+IC
oz
=-IT+
IC
[£-
ICQ-
I12J
2
-
IC
[£-
ICQ-
I12J
8Q
2Q
2
IT+
JC
Q IT+
JC
(1)
- 1 n
[ICTI-
2A.A(Il+
I12-
A.z

]2
= 0
2(IC + TI) Q
(I') {
IC
£ _
ICQ
-
I1
A-
= O
('>
2
(TI-
IC)
" 2 A A ( n +
IC
) -
TI
2
A
2
.
n-
= o
Q2
(1')
(2')
g > 0
Ne'u

(I')
c6
E =
ii:
IC
{-ru
+[ZAAIC(l
+
(IU)'
]}
20
(!b)
I I
Q * = [
IT
+n
JC
]
2
[ 2
A-A
_ (
2)
2
]
2
> O
I1
IC
IC

(
I1
+
JC
)
(2b)
Khi d6:
1 [
('>
]
z(E,Q);:::z(E*,Q*)=f(Q*)=
('>
ICTIQ*+ *
+2Im'A
2(I1+Iq
Q
=
('>
1
(2
1ciT[
2M(Ic
+IT)-
TI
2
2
2
J +
21au]
2(I1+

!C)
Con
ne'u
nhu' thuc ( 1 b) (2b) khong c6 nghia
m()t
trong
hai
thuc c6 gia
tri
khong duong thi z(E,Q) gia
tri
nho nha't khi
E*
= 0.
Trang tru'ong hqp
d6 ta trd ph'an A va c6 cong thuc Willson.
nhien khi
IT
> 0 va r = 0 thi
E*
> 0.
Trang 18
Ktt
lu{ln:
- Ne'u n = 0, n 8 thl z(E,Q)
d<;tt
gia tri nho nha't khi E = +
00.
Q =
+oo

va
Zmin
=
TIA.
- Ne'u
E*'
Q*
xac
dinh bdi cong thuc
(1
b) (2b) c6 nghia va c6 gia
tq
du'dng thl z(E,Q)
d<;tt
gia
tri
nho nha't
t<;ti
(E*,
Q*).
-
Ne'u cong thuc
(lb),
(2b) khong c6 nghia gia
tri
khong
du'dng va ll
-:F
0 thl
Z(E,Q)

d<;tt
gia
tri
nho nhfft khi
E*
= 0
Va
Q* xac
dinh
cong thuc Willson.
Thui
diem
diit hang:
Bay
giG
ta n6i de'n thoi di6m hang.
Md
r(mg ta c6 th6 cho
phep
lu'qng t6n kho gia
tri
am.
T<;ti
thoi di6m t ba't ky
tn
t
tn+
I ta dinh nghia
luqng dlf
tru

t6n kho la:
S(t) = Q* -
E*
- A(t-
tn)·
Ne'u S(t) < 0
thll
S(t) I chinh la lu'qng hang
yeu
diu
de'n thoi di6m d6
chua du'qc dap ung.
Tu'dng tlf nhu' d ph'an A,
t<;ti
thoi di6m hang lu'qng dlf
tru
t6n kho
t6ng
quat
se
la:
r* =
! l
- mQ* -
E*.
Trong d6
! l
=A
't.
m

=['tiT]
Th£
d1;1:
Xet
thong
quan
ly dlf
tru
cho voi cac
tham
so:
A
= 200 ddn vi I
nam
A=
50.000
d.
I=
0,20
n = 2.000 d/ ddn vi.
c = 250.000 d
n = 100.000
d/
ddn vi
nam
't
= 9 thang
Trang 19
T * = = o
12

nam
200 '
[
't
] [ 9 ]
m
-6
T *
12
.0,12
11
="A
't
= 200 x 9/12 = 200 x 0,75 = 150 don vi.
r* =
11-
mQ* - s* =
150- 144-
5 = 1 don vi.
ta hang thai
diem
trong khi dlf
tru
con 1 don vi.
Va
m6i l'an
voi
khO"i
lu'<;Jng
la 24 don vi.

Nhu' chi phi blnh
quan
hang
nam
se
la:
z = 1.235.338.
c.
TRUONG
HQP
MAT
YEU CAU,
KHI
YEU CAU KHONG
DUOC
DAP
UNG:
Ph'an tru'oc ta gia thi€t
ding
nhung
yeu
c'a
u d€n
ht%
th6ng khi khong
con hang dlf
tru
thl nhung
yeu
c'au d6

se
cha.
Bay
gia
ta
xet
tru'ang
h<;Jp
khi
yeu
c'au xua't
hit%n
rna
hang
dlf
tru
khong con thl
yeu
c'au d6 bi ma't. Chung ta
se
chi ra ding trong tru'ang
h<;Jp
nay
lam
t6i thieu chi phi blnh quan hang
nam
va
vit%c
lam
eve

l<;Ji
blnh
quan
hang
nam
cho ta cling m()t k€t
qua. Gia
sli'
P ky
hit%u
gia
ban
cua 1 san ph§:m hang h6a; B la
l<;Ji
blnh
quan hang niim
TI
0
Ia
chi phi
(thit%t
do
sv
ma't
yeu
c'au voi
khO"i
lu'<;Jng
m()t don vi hang
h6a

con C
Ia
gia mua m()t don vi hang h6a.
Khi d6
n€u f
0
ky
hit%u
khoang thai gian trong 1 niim, trong thai gian d6
th6ng thi€u dlf
tru
thll<;Ji
blnh
quan
hang
niim
se
Ia:
B =
"A(P-
C)(1 - fo)-
Tio"Af
0
-
(chi phi
hang
va chi phi
lu'u
kho).
=

"A(P-
C)-
[Tio
+
P-
C]"Af
0
-
(chi phi
hang
va
chi phi
lu'u
kho).
Trang 20
d day ta c6 the
A(P
- C)
la
tri
s6
gia tang blnh quan
hang
narn
neu
khong thie'u hang h6a;
• (P -
C)Afo
la trj
s6

chi phi
h;;ti)
c1;1
the blnh quan do thie'u
h1;1t
hang h6a
dv
trii'.
•
I1oAf
0
la
tq
s6
chi phi
h;;ti)
uy tin blnh quan hang narn.
• (P
+ IIo - C)Af
0
la trj
s6
chi phi t6ng
quat
blnh quan do thie'u
h1;1t
hang
d\1'
tru.
TI

=
I1o
+ P -
C.
Ta c6
hieu
TI
la
tq
s6
chi phi t6ng
quat
cho
yeu
c'au 1 don vj hang h6a bj rna't, cling c6
the
hieu n chinh
Ia
lqi thu duqc
khi ta dap ling nhu c'au 1 don vi hang h6a, so voi khi chung ta khong dap ling
du trong kho hang h6a con.
Gil:1
sll'
T la khming thoi gian thie'u
h1;1t
hang
dl;l'
trii' trong rn9t chu
ky
va Q

la
luqng hang b6 sung cho rn6i rn9t l'an
b6
sung. d9
IOn
cua rn9t
chu
ky
la: T = (Q/A) +
T.
T6ng chi phi trong rn9t chu ky la:
IC Q
2
A+ +TIAT
2 A
s6 chu
ky
blnh quan hang narn la:
1 A
= , ,-
T
Q+AT
chi phi blnh quan hang narn
se
Ia:
AA
IC Q
2
TIA.AT
Z=

Q+AT
2
Q+AT
Q+AT
z
=ITA.+
ICQ
2
-
2ITA.q_
+
2AA.
2(Q+A.T)
(1)
Theo
c6ng thlic (
1)
ta tha'y
ham
s6
z(Q, T ) don theo T rn6i khi ta
c6
djnh Q. z(Q, T ) chi c6 the
d;;tt
gua
trj nho nha't khi T = 0 T = +
oo
Ta't nhien khi T =
+oo
thl z(Q,+

oo)
=
TIA
Trang
21
V
, •
d""
kh"
a xay ra
au
= 1 Q * =

IC
)N
K I
a
eu
ITA::;;
-v2AIC
A
<=>
IT
<
-A-
Ttic Ia chi phi do ma't
mot
yeu
c'au nho hon chi phi
hang

va htu kho
binh quan
to'i
thi€u
tren
1 don vi hang h6a thi z
d(!.t
gia
tri
nho nha't
t(!.i
T =
+oo
khi d6 ta khong c'an tho'ng
dl,l'
trii'
tuc Ia tho'ng ngung
ho(!.t
d()ng.
b) Khi
Tuc Ia khi chi phi binh
quando
ma't m()t
yeu
c'au 1 don vi hang
h6a
IOn
hon chi phi
hang
va htu kho binh

quan
to'i
thi€u hang
nam
tren 1 don
vi hang
h6a thi z
d(!.t
gia
tq
nho nha't khi T = o tuc Ia ta khong
bao
giO
d€
tho'ng thie'u hang
dl,l'
trii'.
Do
d6 ta
l(!.i
c6 cong thuc Wilson.
Trang 22