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Chương I-Bài 2. Cực trị hàm số
CỰC TRỊ CỦA HÀM SỐ
§BÀI 2.
A. LÝ THUYẾT.
1. Khái niệm cực trị hàm số :
Giả sử hàm số xác định trên tập hợp D D
và x0 D
x0 được gọi là một điểm cực đại của hàm số f nếu tồn tại một khoảng a; b chứa điểm x0
sao cho:
a; b D
.
f ( x) f ( x0 ) x a; b \ x0
Khi đó f x0 được gọi là giá trị cực đại của hàm số f .
x0 được gọi là một điểm cực tiểu của hàm số f nếu tồn tại một khoảng a; b chứa điểm x0
sao cho:
a; b D
.
f
(
x
)
f
(
x
)
x
a
;
b
\
x
0
0
Khi đó f x0 được gọi là giá trị cực tiểu của hàm số f .
Giá trị cực đại và giá trị cực tiểu được gọi chung là cực trị
Nếu x0 là một điểm cực trị của hàm số f thì người ta nói rằng hàm số f đạt cực trị tại điểm x0 .
Điểm cực đại, cực tiểu gọi chung là điểm cực trị của hàm số
f x0 là giá trị cực trị (hay cực trị ) của hàm số.
Như vậy : Điểm cực trị phải là một điểm trong của tập hợp D
``Chú ý.
Giá trị cực đại (cực tiểu) f x0 của hàm số f chưa hẳn đã là GTLN (GTNN) của hàm số f trên tập
xác định D mà f x0 chỉ là GTLN (GTNN) của hàm số f trên khoảng a; b D và a; b chứa
điểm x0 .
Nếu f x không đổi dấu trên tập xác định D của hàm số f thì hàm số f không có cực trị .
2. Điều kiện cần để hàm số đạt cực trị:
2.1. Định lý 1: Giả sử hàm số f đạt cực trị tại điểm x0 .
Khi đó, nếu f có đạo hàm tại điểm x0 thì f ' x0 0 .
Chú ý :
Đạo hàm f ' có thể triệt tiêu tại điểm x0 nhưng hàm số f không đạt cực trị tại điểm x0 .
Hàm số có thể đạt cực trị tại một điểm mà tại đó hàm số không có đạo hàm.
Hàm số chỉ có thể đạt cực trị tại một điểm mà tại đó đạo hàm của hàm số bằng 0, hoặc tại
đó hàm số không có đạo hàm .
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Chương I-Bài 2. Cực trị hàm số
3. Điều kiện đủ để hàm số đạt cực trị:
Định lý 2: Giả sử hàm số f có đạo hàm cấp một trên khoảng a; b chứa điểm x0 , f ' x0 0 và f
có đạo hàm cấp hai khác 0 tại điểm x0 .
Nếu f '' x0 0 thì hàm số f đạt cực đại tại điểm x0 .
Nếu f '' x0 0 thì hàm số f đạt cực tiểu tại điểm x0 .
Chú ý :
Nếu x0 là một điểm cực trị của hàm số f thì điểm ( x0 ; f ( x0 )) được gọi là điểm cực trị của đồ
thị hàm số f .
f '( x0 ) 0
Trong trường hợp f '( x0 ) 0 không tồn tại hoặc
thì định lý 3 không dùng được.
f ''( x0 ) 0
B. PHƯƠNG PHÁP GIẢI TOÁN.
DẠNG 1. Tìm các điểm cực trị của hàm số.
1. Phương pháp.
① Bước 1. Tìm tập xác định của hàm số f .
② Bước 2. Tính đạo hàm f ( x ) và tìm các điểm x0 sao cho f ( x0 ) = 0 (nếu có) và tìm các điểm
x0 D mà tại đó hàm f liên tục nhưng đạo hàm f ( x) không tồn tại.
③ Bước 3. Vận dụng định lý 2 (lập bảng xét dấu f ( x ) ) hay định lý 3( tính f ( x) ) để xác định
điểm cực trị của hàm số.
⋆ Chú ý:
Cho hàm số y f ( x) xác định trên D . Điểm x x0 D là điểm cực trị của hàm số khi và chỉ khi
hai điều kiện sau đây cùng thảo mãn:
Tại x x0 đạo hàm triệt tiêu hoặc không tồn tại
Đạo hàm đổi dấu khi x đi qua x0 .
2. Bài tập minh họa.
Bài tập 1. Tìm cực trị (nếu có) của các hàm số sau:
1). y x 4 2 x 2 1
2). y x 4 6 x 2 8 x
Lời giải.
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Chương I-Bài 2. Cực trị hàm số
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y '(1) 0
Nhận xét . Trong bài toán này, vì
do đó định lý 3 không khẳng định được điểm x 2 có
y ''(1) 0
phải là điểm cực trị của hàm số hay không.
Bài tập 2. Tìm cực trị (nếu có) của các hàm số sau:
3
1). y x3 x 2 6 x 1
2). y x x 2 x 1
2
Lời giải.
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Bài tập 3. Tìm cực trị (nếu có) của các hàm số sau:
4 x
1). y
4 x
2). y x 3
1
x 1
Lời giải.
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Chương I-Bài 2. Cực trị hàm số
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Bài tập 4. Tìm cực trị (nếu có) của hàm số : y 3 2 cos x cos 2 x
Lời giải.
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Chương I-Bài 2. Cực trị hàm số
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1
2
x sin , x 0
Bài tập 5. Cho hàm số f x
. Chứng minh rằng f ' x 0 nhưng hàm số f x
x
0
, x0
không đạt cực trị tại điểm 0 .
Lời giải.
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3. Câu hỏi trắc nghiệm
Mức độ 1. Nhận biết
Câu 1. Cho hàm số y x 3 3 x. Mệnh đề nào dưới đây đúng?
A. Hàm số đồng biến trên khoảng ; 1 và nghịch biến trên khoảng 1; .
B. Hàm số đồng biến trên khoảng (; ).
C. Hàm số nghịch biến trên khoảng ; 1 và đồng biến trên khoảng 1;
D. Hàm số nghịch biến trên khoảng 1;1 .
Lời giải
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Câu 2.(THPT Chuyên Bắc Ninh 2018) Phát biểu nào sau đây là sai?
A. Nếu f x0 0 và f x0 0 thì hàm số đạt cực tiểu tại x0 .
B. Nếu f x0 0 và f x0 0 thì hàm số đạt cực đại tại x0 .
C. Nếu f x đổi dấu khi x qua điểm x0 và f x liên tục tại x0 thì hàm số y f x đạt cực
trị tại điểm x0 .
D. Hàm số y f x đạt cực trị tại x0 khi và chỉ khi x0 là nghiệm của đạo hàm.
Lời giải
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Chương I-Bài 2. Cực trị hàm số
Câu 3.(THPT Bình Xuyên-Vĩnh Phúc 2018) Xét f x là một hàm số tùy ý. Khẳng định nào sau đây
là khẳng định đúng?
A. Nếu f x đạt cực tiểu tại x x0 thì f x0 0 .
B. Nếu f x0 0 thì f x đạt cực trị tại x x0 .
C. Nếu f x0 0 và f x0 0 thì f x đạt cực đại tại x x0 .
D. Nếu f x có đạo hàm tại x0 và đạt cực đại tại x0 thì f x0 0 .
Lời giải
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Câu 4.(THPT Chuyên Quốc Học Huế 2018) Cho hàm số y f x có đạo hàm cấp 2 trên khoảng
K và x0 K . Mệnh đề nào sau đây đúng ?
A. Nếu f x 0 thì x0 là điểm cực tiểu của hàm số y f x .
B. Nếu f x 0 thì x0 là điểm cực trị của hàm số y f x .
C. Nếu x0 là điểm cực trị của hàm số y f x thì f x0 0 .
D. Nếu x0 là điểm cực trị của hàm số y f x thì f x0 0 .
Lời giải
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Câu 5.(THPT Chuyên Quốc Học Huế) Cho hàm số f x có đạo hàm cấp 2 trên khoảng K và
x0 K . Tìm mệnh đề sai trong các mệnh đề sau:
A. Nếu hàm số đạt cực đại tại x0 thì f x0 0 .
B. Nếu hàm số đạt cực đại tại x0 thì tồn tại a x0 để f a 0 .
C. Nếu hàm số đạt cực trị tại x0 thì f x0 0 .
D. Nếu f x0 0 và f x0 0 thì hàm số đạt cực trị tại x0 .
Lời giải
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Câu 6.(THPT Chuyên Hùng Vương 2018) Cho hàm số y f x có đạo hàm trên
. Xét tính đúng
sai của các mệnh đề sau:
(I): Nếu f x 0 trên khoảng x0 h; x0 và f x 0 trên khoảng x0 ; x0 h h 0 thì hàm
số đạt cực đại tại điểm x0 .
(II): Nếu hàm số đạt cực đại tại điểm x0 thì tồn tại các khoảng x0 h; x0 , x0 ; x0 h h 0 sao
cho f x 0 trên khoảng x0 h; x0 và f x 0 trên khoảng x0 ; x0 h .
A. Cả (I) và (II) cùng sai.
C. Mệnh đề (I) sai, mệnh đề (II) đúng.
B. Mệnh đề (I) đúng, mệnh đề (II) sai.
D. Cả (I) và (II) cùng đúng.
Lời giải
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Chương I-Bài 2. Cực trị hàm số
Câu 7.(THPT Chuyên Hùng Vương-Phú Thọ 2018) Điểm cực tiểu của đồ thị hàm số y x 3 3 x 5
là điểm ?
A. Q 3; 1 .
B. M 1; 3 .
C. P 7; 1 .
D. N 1; 7 .
Lời giải
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Câu 8.(Chuyên Đồng Bằng Sông Cửu long2018) Gọi x1 là điểm cực đại, x2 là điểm cực tiểu của
hàm số y x3 3 x 2 . Tính x1 2 x2 .
A. 2 .
B. 1 .
C. 1 .
D. 0 .
Lời giải
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Câu 9.(TT Diệu Hiền-Cần Thơ 2018) Hàm số y x3 3 x 2 3 x 4 có bao nhiêu cực trị?
A. 1 .
B. 2 .
C. 0 .
D. 3 .
Lời giải
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Câu 10.(THPT Chuyên Vĩnh Phúc 2018) Tìm giá trị cực đại yCĐ của hàm số y x 3 12 x 1
A. yCĐ 17 .
B. yCĐ 2 .
C. yCĐ 45 .
D. yCĐ 15 .
Lời giải
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Câu 11.(THPT Triệu Sơn 3 Thanh Hóa 2018) Có bao nhiêu điểm cực trị của hàm số y
A. 3 .
B. 2 .
C. 0 .
1
?
x
D. 1
Lời giải
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Câu 12.(Sở GD & ĐT Bình Thuận 2020) Cho hàm số y x 4 2 x 2 1 có giá trị cực đại và giá trị
cực tiểu lần lượt là y1 và y2 . Khi đó, khẳng định nào sau đây đúng?
A. 3 y1 y2 1 .
B. 3 y1 y2 5 .
C. 3 y1 y2 1 .
D. 3 y1 y2 5 .
Lời giải
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Chương I-Bài 2. Cực trị hàm số
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Câu 13.(THPT Chuyên Vĩnh Phúc-2018) Hàm số y x 4 2 x 2 3 có bao nhiêu điểm cực trị?
A. 0 .
B. 2 .
C. 1 .
D. 3 .
Lời giải
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Câu 14.(THPT Chuyên Hạ Long 2018) Hàm số y x 4 2 x 2 5 có bao nhiêu điểm cực trị?
A. 1 .
B. 3 .
C. 0 .
D. 2 .
Lời giải
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Câu 15.(THPT Trần Quốc Tuấn 2018) Hàm số y 2 x 4 4 x 2 8 có bao nhiêu điểm cực trị?
A. 2 .
B. 4 .
C. 3 .
D. 1 .
Lời giải
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Câu 16.(THPT Chuyên Hà Tĩnh 2018) Số điểm cực trị của đồ thị hàm số y x 4 2 x 2 2 là
A. 2 .
B. 3 .
C. 0 .
D. 1 .
Lời giải
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Câu 17.(THPT Hồng Bàng 2018) Cho hàm số y f x có đạo hàm là f x x x 1 x 1 .
2
Hàm số y f x có bao nhiêu điểm cực trị?
A. 1 .
B. 2 .
C. 0 .
D. 3 .
Lời giải
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Chương I-Bài 2. Cực trị hàm số
Câu 18.(Chuyên Quang Trung-2018) Cho các hàm số
I : y x 2 3 , II : y x3 3x 2 3x 5 ,
1
7
, IV : y 2 x 1 . Các hàm số không có cực trị là:
x2
A. I , II , III .
B. III , IV , I .
III : y x
C. IV , I , II .
D. II , III , IV .
Lời giải
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Câu 19.(THPT Nguyễn Khuyến-Nam Định 2018) Đồ thị hàm số nào trong bốn hàm số liệt kê ở bốn
phương án A, B, C, D dưới đây, có đúng một cực trị?
2x 3
A. y x 3 3x 2 x .
B. y x 4 2 x 2 3 .
C. y x3 4 x 5 .
D. y
.
x 1
Lời giải
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Câu 20.(THPT Can Lộc Hà Tĩnh 2018) Trong các hàm số sau, hàm số nào có hai điểm cực đại và
một điểm cực tiểu?
A. y x 4 x 2 3 .
B. y x 4 x 2 3 .
C. y x 4 x 2 3 .
D. y x 4 x 2 3
Lời giải
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Câu 21.(THPT Chuyên Thái Bình 2018)
Hàm số y
A. 3 .
3
x
2
2 x 3 2 có tất cả bao nhiêu điểm cực trị
2
B. 0 .
C. 1 .
D. 2 .
Lời giải
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Chương I-Bài 2. Cực trị hàm số
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Câu 22.(THPT Hồng Bàng Hải Phòng 2018) Hàm số y 4 x 2 có bao nhiêu điểm cực tiểu?
A. 1 .
B. 0 .
C. 3 .
D. 2 .
Lời giải
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Câu 23.(Sở GD & ĐT Hậu Giang 2020) Đồ thị hàm số nào sau đây có đúng 1 điểm cực trị
A. y x3 6 x 2 9 x 5 .
B. y x 4 3x 2 4 .
C. y x 3 3 x 2 3 x 5 .
D. y 2 x 4 4 x 2 1 .
Lời giải
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Mức độ 2. Thông hiểu
Câu 24.(THPT Hoa Lư-2018) Gọi A và B là các điểm cực tiểu của đồ thị hàm số y x 4 2 x 2 1.
Tính diện tích S của tam giác OAB ( O là gốc tọa độ)
A. S 2 .
B. S 4 .
C. S 1 .
D. S 3 .
Lời giải
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Câu 25.(THPT Sơn Tây-Hà Nội-2018) Viết phương trình đường thẳng đi qua hai điểm cực trị của
x2 2x
đồ thị hàm số y
x 1
A. y 2 x 2 .
B. y 2 x 2 .
C. y 2 x 2 .
D. y 2 x 2 .
Lời giải
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Chương I-Bài 2. Cực trị hàm số
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Câu 26.(THPT Sơn Tây-Hà Nội-2018) Tìm cực đại của hàm số y x 1 x 2 .
1
1
1
1
A.
B.
.
C. .
D. .
2
2
2
2
Lời giải
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Câu 27.(THPT Chuyên ĐHSP-2018) Điểm thuộc đường thẳng d : x y 1 0 cách đều hai điểm cực
trị của đồ thị hàm số y x 3 3 x 2 2 là
A. 2;1 .
B. 0; 1 .
C. 1;0 .
D. 1; 2 .
Lời giải
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Câu 28.(Chuyên Phan Bội Châu-2018) Số điểm cực trị của hàm số y x 1 3 x 2 là
A. 1 .
B. 2 .
C. 3 .
D. 0 .
Lời giải
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Câu 29.(THPT Chuyên Lê Qúy Đôn 2020)
Cho hàm số f x có đạo hàm f x x 1 x 2 3 x 4 1 trên
. Tính số điểm cực trị của
hàm số y f x .
A. 2 .
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B. 3 .
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D. 4 .
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Chương I-Bài 2. Cực trị hàm số
Lời giải
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Câu 30.(THPT Phan Đăng Lưu Huế 2020) Gọi A , B là hai điểm cực trị của đồ thị hàm số
f x x3 3x 4 và M x0 ;0 là điểm trên trục hoành sao cho tam giác MAB có chu vi nhỏ
nhất, đặt T 4 x0 2015 . Trong các khẳng định dưới đây, khẳng định nào đúng ?
A. T 2017 .
B. T 2019 .
C. T 2016 .
D. T 2018 .
Lời giải
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Câu 31.(THPT Trần Phú 2018)
Cho hàm số y x 4 8 x 2 10 có đồ thị C . Gọi A , B , C là 3 điểm cực trị của đồ thị C . Tính
diện tích S của tam giác ABC .
A. S 64 .
B. S 32 .
C. S 24 .
D. S 12 .
Lời giải
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Chương I-Bài 2. Cực trị hàm số
DẠNG 2. Định tham số m để hàm số f x đạt cực trị.
Loại 1. Định tham số m để hàm số f x đạt cực trị tại điểm x0 cho trước.
1. Phương pháp.
① Bước 1. Tìm tập xác định của hàm số f và tính đạo hàm f ( x )
② Bước 2. Điều kiện cần để hàm số đạt cực trị tại x0 là y '( x0 ) 0 , từ điều kiện này ta tìm được giá
trị của tham số m .
③ Bước 3. Kiểm lại bằng cách dùng một trong hai quy tắc tìm cực trị, để xét xem giá trị của tham
số m vừa tìm được có thỏa mãn yêu cầu của bài toán hay không ?
⋆ Chú ý:
⋇ Ta có thể sử dụng quy tắc hai để tìm, tuy nhiên việc sử dụng quy tắc hai phải thỏa mãn điều
kiện y ''( x0 ) 0 .
⋇ Giả sử hàm số f có đạo hàm cấp một trên khoảng a; b chứa điểm x0 , f ' x0 0 và f có đạo
hàm cấp hai khác 0 tại điểm x0 .
Nếu f x0 0 thì hàm số f đạt cực đại tại điểm x0 .
Nếu f x0 0 thì hàm số f đạt cực tiểu tại điểm x0 .
2. Bài tập minh họa.
1
Bài tập 6. Cho hàm số y x3 mx 2 m2 m 1 x 1 . Với giá trị nào của m thì hàm số đạt cực
3
đại tại điểm x 1 .
Lời giải.
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Chú ý:
f '( x0 ) 0
thì định lý 3 không dùng được.
f ''( x0 ) 0
Trong trường hợp f '( x0 ) 0 không tồn tại hoặc
Nhận xét:
y '(1) 0
Nếu trình bày lời giải theo sơ đồ sau: Hàm số đạt cực đại tại x 1
thì lời giải
y ''(1) 0
chưa chính xác
Vì dấu hiệu nêu trong định lí 3 chỉ phát biểu khi y ''( x0 ) 0 . Các bạn sẽ thấy điều đó rõ hơn bằng
cách giải bài toán sau:
4
2
2
1). Tìm m để hàm số y x 3mx m m đạt cực tiểu tại x 0
3
2
2). Tìm m đề hàm số y x 3(m 2) x (m 4) x 2m 1 đạt cực đại tại x 1 .
Nếu ta khẳng định được y ''( x0 ) 0 thì ta sử dụng được.
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Chương I-Bài 2. Cực trị hàm số
Bài tập 7. Tìm m để hàm số:
x3
1). y (2m 1) x 2 (m 9) x 1 đạt cực tiểu tại x 2 .
3
2). y mx3 2(m 1) x 2 (m 2) x m đạt cực tiểu tại x 1 .
x 2 mx 1
đạt cực tiểu tại x 1 .
xm
x 2 (m 1) x 3 2m
4). y
đạt cực đại tại x 1 .
xm
Lời giải.
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3). y
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Chương I-Bài 2. Cực trị hàm số
3. Câu hỏi trắc nghiệm
Mức độ 1. Nhận biết
Câu 32.(THPT Nguyễn Đức Thuận 2018) Tìm m để hàm số y x 4 2mx 2 2m m 4 5 đạt cực
tiểu tại x 1 .
A. m 1 .
B. m 1 .
C. m 1 .
D. m 1 .
Lời giải
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Câu 33.(THPT Tam Phước 2018) Với giá trị nào của tham số m thì hàm số
1
y x3 mx 2 m2 m 1 x 1 đạt cực đại tại điểm x 1 .
3
A. m 2 .
B. m 3 .
C. m 1 .
D. m 0 .
Lời giải
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Câu 34.(THPT Kiến An 2018) Tìm tất cả các giá trị thực của tham số m để hàm số
y mx3 x 2 m 2 6 x 1 đạt cực tiểu tại x 1 .
A. m 1 .
B. m 4 .
C. m 2 .
D. m 2 .
Lời giải
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Chương I-Bài 2. Cực trị hàm số
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Câu 35.(THPT Hà Huy Tập 2018) Tìm giá trị thực của tham số m để hàm số
1
y x3 mx 2 m2 4 x 3 đạt cực tiểu tại x 3 .
3
A. m 1 .
B. m 1 .
C. m 5 .
D. m 7 .
Lời giải
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Câu 36.(THPT Xuân Hòa 2018) Hàm số y x 3 3 x 2 mx 2 đạt cực tiểu tại x 2 khi:
A. m 0 .
B. m 0 .
C. m 0 .
D. m 0 .
Lời giải
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Câu 37.(THPT Việt Trì 2018) Hàm số y x3 3 m 1 x 2 3 m 1 x . Hàm số đạt cực trị tại điểm
2
có hoành độ x 1 khi
A. m 1 .
B. m 0; m 4 .
C. m 4 .
D. m 0; m 1 .
Lời giải
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Câu 38.(THPT Chuyên Lê Quý Đôn 2018) Cho hàm số f x x3 3mx 2 3 m 2 1 x . Tìm tất cả các
giá trị của m để hàm số f x đạt cực đại tại x0 1 .
A. m 0 và m 2 .
B. m 2 .
C. m 0 .
D. m 0 hoặc m 2 .
Lời giải
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Chương I-Bài 2. Cực trị hàm số
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Câu 39.(THPT Quãng Xương 2018) Đồ thị hàm số y x 3 3x 2 2ax b có điểm cực tiểu A 2; 2 .
Tính a b .
A. a b 4 .
B. a b 2 .
C. a b 4 .
D. a b 2 .
Lời giải
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Câu 40.(THPT Trần Hưng Đạo 2018) Tìm tất cả giá trị thực của tham số m để hàm số
y x 4 2(m 1) x 2 m2 1 đạt cực tiểu tại x 0 .
A. m 1 .
B. m 1 .
C. m 1 .
D. m 1 m 1
Lời giải
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Câu 41.(THPT Xuân Trường 2018) Hàm số y x 4 2mx 2 1 đạt cực tiểu tại x 0 khi:
A. 1 m 0.
B. m 0.
C. m 1.
D. m 0.
Lời giải
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Câu 42.(THPT Hoài Ân 2018) Tìm giá trị thực của tham số m để hàm số
1
y x3 mx 2 m2 4 x 3 đạt cực đại tại điểm x 3 .
3
A. m 7 .
B. m 5 .
C. m 1 .
D. m 1 .
Lời giải
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Chương I-Bài 2. Cực trị hàm số
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Câu 43.(THPT Chuyên Biên Hòa 2018) Hàm số y x 3 2ax 2 4bx 2018 , a, b
đạt cực trị tại
x 1 . Khi đó hiệu a b là
A. 1 .
B.
4
.
3
C.
3
.
4
3
D. .
4
Lời giải
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Câu 44.(SGD Bà Rịa Vũng Tàu 2018)
1
Tìm giá trị thực của tham số m để hàm số y x3 mx 2 m2 m 1 x đạt cực đại tại x 1 .
3
A. m 2 .
B. m 3 .
C. m .
D. m 0 .
Lời giải
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Câu 45.(Sở GD 7 ĐT Bắc Ninh 2018)
1
3
3
Tìm giá trị của tham số m để hàm số y x
A. m 2 .
B. m 2 .
1 2
m 1 x 2 3m 2 x m đạt cực đại tại x 1 ?
2
C. m 1 .
D. m 1 .
Lời giải
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Chương I-Bài 2. Cực trị hàm số
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Câu 46.(THPT Chuyên Lam Sơn 2018) Tìm m để hàm số y mx3 m 2 1 x 2 2 x 3 đạt cực tiểu
tại x 1 .
A. m
3
.
2
3
B. m .
2
C. m 0 .
D. m 1 .
Lời giải
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Câu 47.(Sở GD & ĐT Hà Nội 2018)
Tìm tất cả các giá trị thực của tham số m để hàm số y x 4 mx 2 đạt cực tiểu tại x 0 .
A. m 0 .
B. m 0 .
C. m 0 .
D. m 0 .
Lời giải
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Câu 48.(THPT Chuyên Lam Sơn 2018) Tìm m để hàm số y mx3 m 2 1 x 2 2 x 3 đạt cực tiểu
tại x 1 .
A. m
3
.
2
3
B. m .
2
C. m 0 .
D. m 1 .
Lời giải
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Chương I-Bài 2. Cực trị hàm số
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Câu 49.(Sở GD & ĐT Quãng Nam 2018) Tìm tất cả các giá trị thực của tham số m để hàm số
1
1
y x3 2m 3 x 2 m2 3m 4 x đạt cực tiểu tại x 1 .
3
2
A. m 2 .
B. m 3 .
C. m 3 hoặc m 2 .
D. m 2 hoặc m 3 .
Lời giải
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Mức độ 2. Thông Hiểu
Câu 50.(THPT Nguyễn Khuyến 2018) Để hàm số y
khoảng nào?
A. 2; 4 .
B. 0; 2 .
x 2 mx 1
đạt cực đại tại x 2 thì m thuộc
xm
C. 4; 2 .
D. 2; 0 .
Lời giải
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Trung Tâm Luyện Thi Đại Học Amsterdam
Chương I-Bài 2. Cực trị hàm số
Câu 51.(THPT Thạch Thành 2018) Cho hàm số y x 4 ax 2 b . Biết rằng đồ thị hàm số nhận
điểm A 1; 4 là điểm cực tiểu. Tổng 2a b bằng
A. 1 .
B. 0 .
C. 1 .
D. 2 .
Lời giải
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Câu 52.(THPT Thạch Thành 2018)
Đồ thị hàm số y ax 3 bx 2 cx d có hai điểm cực trị là A 1; 7 , B 2; 8 . Tính y 1 .
A. y 1 11 .
B. y 1 7 .
C. y 1 11 .
D. y 1 35 .
Lời giải
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Câu 53.(THPT Chuyên Phan Bội Châu 2018) Biết điểm M 0; 4 là điểm cực đại của đồ thị hàm số
f x x3 ax 2 bx a 2 . Tính f 3 .
A. f 3 17 .
B. f 3 49 .
C. f 3 34 .
D. f 3 13 .
Lời giải
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Câu 54.(THPT Đức Thọ Hà Tĩnh 2018) Xác định các hệ số a , b , c để đồ thị hàm số
y ax 4 bx 2 c , biết điểm A 1; 4 , B 0; 3 là các điểm cực trị của đồ thị hàm số.
A. a 1 ; b 0 ; c 3 .
C. a 1 ; b 3 ; c 3 .
1
B. a ; b 3 ; c 3 .
4
D. a 1 ; b 2 ; c 3 .
Lời giải
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Chương I-Bài 2. Cực trị hàm số
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Câu 55.(Đề Chính Thức Bộ Giáo Dục 2018) Có tất cả bao nhiêu giá trị nguyên của m để hàm số
y x8 m 2 x5 m 2 4 x 4 1 đạt cực tiểu tại x 0.
A. 3 .
B. 5 .
C. 4 .
D. Vô số.
Lời giải
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Câu 56.(Đề Chính Thức Bộ Giáo Dục 2018) Có bao nhiêu giá trị nguyên của tham số m để hàm
số y x8 (m 1) x 5 (m 2 1) x 4 1 đạt cực tiểu tại x 0?
A. 3 .
B. 2 .
C. Vô số.
D. 1 .
Lời giải
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Câu 57.(Đề Chính Thức Bộ Giáo Dục 2018) Có bao nhiêu giá trị nguyên của tham số m để hàm số
y x8 m 4 x5 m 2 16 x 4 1 đạt cực tiểu tại x 0 .
A. 8 .
B. Vô số.
C. 7 .
D. 9 .
Lời giải
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Chương I-Bài 2. Cực trị hàm số
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Câu 60.(THPT Kim Liên Hà Nội 2018)Cho hàm số y x3 2 x 2 ax b , a, b
có đồ thị C .
Biết đồ thị C có điểm cực trị là A 1;3 . Tính giá trị của P 4a b .
A. P 3 .
B. P 2 .
C. P 4 .
D. P 1 .
Lời giải
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Câu 61.(THPT Chuyên Quốc Học Huế 2020) Cho hàm số f x x3 ax 2 bx c đạt cực tiểu tại
điểm x 1 , f 1 3 và đồ thị hàm số cắt trục tung tại điểm có tung độ bằng 2 . Tính T a b c
A. T 9 .
B. T 1 .
C. T 2 .
D. T 4 .
Lời giải
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Câu 62.(Toán Học Tuổi Trẻ 2017) Đồ thị hàm số y x 3 3x 2 2ax b có điểm cực tiểu A 2; 2 .
Khi đó a b bằng
A. 4 .
B. 2 .
C. 4 .
D. 2 .
Lời giải
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Chương I-Bài 2. Cực trị hàm số
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Câu 63.(Sở GD & ĐT Quảng Nam 2018) Tìm tất cả các giá trị thực của tham số m để hàm số
1
1
y x3 2m 3 x 2 m2 3m 4 x đạt cực tiểu tại x 1 .
3
2
A. m 2 .
B. m 3 .
C. m 3 hoặc m 2 .
D. m 2 hoặc m 3
Lời giải
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Câu 64.(Sở GD&ĐT Bình Phước) Đồ thị hàm số y ax 3 bx 2 cx d có hai điểm cực trị A 1; 7 ,
B 2; 8 . Tính y 1 .
A. y 1 7 .
B. y 1 11 .
C. y 1 11 .
D. y 1 35 .
Lời giải
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Câu 65.Cho biết hàm số y f x x3 ax 2 bx c đạt cực trị tại điểm x 1 , f 3 29 và đồ thị
hàm số cắt trục tung tại điểm có tung độ là 2 . Tính giá trị của hàm số tại x 2 .
A. f 2 4 .
B. f 2 24 .
C. f 2 2 .
D. f 2 16 .
Lời giải
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Câu 66.(THPT Ngô Sĩ Liên Bắc Giang 2018) Biết rằng đồ thị của hàm số y ax 3 bx 2 cx d có
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Chương I-Bài 2. Cực trị hàm số
hai điểm cực trị là 0;0 và 1;1 . Các hệ số a , b , c , d lần lượt là
A. 2; 0; 3; 0 .
B. 2; 3; 0; 0 .
C. 2; 0; 0; 3 .
D. 0; 0; 2; 3 .
Lời giải
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Câu 67.(THPT Chuyên Hoàng Văn Thụ 2019) Có bao nhiêu giá trị nguyên của m thuộc khoảng
m 1 5 m 2 4
x
x m 5 đạt cực đại tại x 0?
2019; 2019 để hàm số y
5
4
A. 110 .
B. 2016 .
C. 100 .
D. 10 .
Lời giải
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Câu 68.(THPT Chuyên Huỳnh Mẫn Đạt 2019) Cho hàm số y
x5
5
0?
2m 1 x 4
m 3
x
3
2019 . Có
bao nhiêu giá trị của tham số m để hàm số đạt cực tiểu tại x
A.Vô số .
B.1 .
C.2 .
D.0 .
Lời giải
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