Chapter 5
Solving for the Unknown: A
Howto Approach for
Solving Equations
McGrawHill/Irwin
©2011 The McGrawHill Companies, All Rights Reserved
#5
Solving for the Unknown: A howto
Approach for Solving Equations
Learning Unit Objectives
LU5.1 Solving Equations for the Unknown
1. Explain the basic procedures used to
solve equations for the unknown
2. List the five rules and the mechanical
steps used to solve for the unknown in
seven situations; know how to check the
answers
52
#5
Solving for the Unknown: A howto
Approach for Solving Equations
Learning Unit Objectives
LU5.2 Solving Word Problems for the Unknown
1. List the steps for solving word problems
2. Complete blueprint aids to solve word
problems; check the solutions
53
Terminology
Expression – A meaningful combination of
numbers and letters called terms.
Equation – A mathematical statement with an
equal sign showing that a mathematical
expression on the left equals the mathematical
expression on the right.
Formula – An equation that expresses in symbols
a general fact, rule, or principle.
54
Variables and constants are terms of
mathematical expressions.
Solving Equations for the Unknown
Equality in equations
A + 8
Left side of equation
58
Right side of equation
Dick’s age in 8 years will equal 58
55
Variables and Constants Rules
1. If no number is in
front of a letter, it is a
1: B = 1B; C = 1C
2. If no sign is in
front of a letter or
number, it is a +: C =
+C; 4 = +4
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Solving for the Unknown Rule
Whatever you do to
one side of an
equation, you must do
to the other side.
57
Opposite Process Rule
If an equation
indicates a process
such as addition,
subtraction,
multiplication, or
division, solve for the
unknown or variable
by using the opposite
process.
58
Opposite Process Rule
A + 8 = 58
8 8
A = 50
Check
50 + 8 = 58
59
Equation Equality Rule
You can add the same
quantity or number to both
sides of the equation and
subtract the same quantity or
number from both sides of the
equation without affecting the
equality of the equation. You
can also divide or multiply
both sides of the equation by
the same quantity or number
(except 0) without affecting
the equality of the equation.
510
Equation Equality Rule
7G = 35
7G = 35
7 7
G = 5
Check
7(5) = 35
511
Multiple Processes Rule
When solving for an
unknown that
involves more than
one process, do the
addition and
subtraction before the
multiplication and
division.
512
Multiple Process Rule
H + 2 = 5
4
H + 2 = 5
4
2 2
H = 3
4
Check
12 + 2 = 5
4
3 + 2= 5
513
()
(4) H = 4(3)
4
H = 12
Parentheses Rule
When equations contain
parentheses (which indicates
grouping together, you solve for
the unknown by first
multiplying each item inside the
parentheses by the number or
letter just outside the
parentheses. Then you continue
to solve for the unknown with
the opposite process used in the
equation. Do the addition and
subtractions first; then the
multiplication and division.
514
Parentheses Rule
5(P 4) = 20
5P – 20 = 20
+20 +20
5P = 40
Check
5(84) = 20
5(4) = 20
20 = 20
515
5P = 40
5 5
P =8
Like Unknown Rule
To solve equations
with like unknowns,
you first combine the
unknowns and then
solve with the
opposite process used
in the equation.
516
Like Unknown Rule
4A + A = 20
5A = 20
5A = 20
5 5
Check
4(4) +4 = 20
16 + 4 = 20
517
A = 4
Solving Word Problems for Unknowns
1) Read the entire Problem
3) Let a variable
represent the unknown
Y = Computers
Read again if necessary
2) Ask: “What is the
problem looking for?”
4) Visualize the
relationship between
the unknowns and
variables. Then set up
an equation to solve
for unknown(s)
4Y + Y = 600
5) Check your results
to ensure accuracy
518
Solving Word Problems for the Unknown
Blueprint aid
Unknown(s)
519
Variable(s)
Relationship
Solving Word Problems for the Unknown
ICM Company sold 4 times as many computers as Ring Company.
The difference in their sales is 27. How many computers of each
company were sold?
Unknown(s)
Variable(s)
Relationship
Cars Sold
ICM 4C 4C
Ring C C
27
4C C = 27
3C = 27
3 3
C = 9
520
Ring = 9 computers
ICM = 4(9) = 36 Computers
Check 36 9 = 27