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Solve: x 18 + x + 6 9 x = 2 3 x .

−2

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Solve: y + 5 5 y + y 15 = 1 y .

−3

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Solve: y y + 6 = 72 y 2 36 + 4 .

Solution

.
Factor all the denominators, so we can note any value ofthe variable that would make any denominator zero. .
Find the least common denominator. The LCD is ( y 6 ) ( y + 6 ) .
Clear the fractions. .
Simplify. .
Simplify. .
Solve the resulting equation. .
.
.
.
.
Check.
y = 6 is an extraneous solution.
.

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Solve: x x + 4 = 32 x 2 16 + 5 .

−4 , 3

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Solve: y y + 8 = 128 y 2 64 + 9 .

−7 , 8

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Solve: x 2 x 2 2 3 x + 3 = 5 x 2 2 x + 9 12 x 2 12 .

Solution

.
We will start by factoring all denominators, to make it easier to identify extraneous solutions and the LCD. .
Note any value of the variable that would make any denominator zero. .
Find the least common denominator.The LCD is 12 ( x 1 ) ( x + 1 )
Clear the fractions. .
Simplify. .
Simplify. .
Solve the resulting equation. .
.
.
.
Check.
x = 1 and x = 1 are extraneous solutions.
The equation has no solution.

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Solve: y 5 y 10 5 3 y + 6 = 2 y 2 19 y + 54 15 y 2 60 .

no solution

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Solve: z 2 z + 8 3 4 z 8 = 3 z 2 16 z 6 8 z 2 + 8 z 64 .

no solution

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Solve a rational equation for a specific variable

When we solved linear equations, we learned how to solve a formula for a specific variable. Many formulas used in business, science, economics, and other fields use rational equations to model the relation between two or more variables. We will now see how to solve a rational equation for a specific variable.

We’ll start with a formula relating distance, rate, and time. We have used it many times before, but not usually in this form.

Solve: D T = R for T .

Solution

.
Note any value of the variable that would make any denominator zero. .
Clear the fractions by multiplying both sides of the equations by the LCD, T . .
Simplify. .
Divide both sides by R to isolate T . .
Simplify. .

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Solve: A L = W for L .

w = A l

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Solve: F A = M for A .

A = F M

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[link] uses the formula for slope that we used to get the point-slope form of an equation of a line.

Solve: m = x 2 y 3 for y .

Solution

.
Note any value of the variable that would make any denominator zero. .
Clear the fractions by multiplying both sides of the equations by the LCD, y 3 . .
Simplify. .
Isolate the term with y . .
Divide both sides by m to isolate y . .
Simplify. .

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Solve: y 2 x + 1 = 2 3 for x .

x = 3 y 8 2

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Solve: x = y 1 y for y .

y = x 1 + x

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Be sure to follow all the steps in [link] . It may look like a very simple formula, but we cannot solve it instantly for either denominator.

Solve 1 c + 1 m = 1 for c .

Solution

.
Note any value of the variable that would make any denominator zero. .
Clear the fractions by multiplying both sides of the equations by the LCD, c m . .
Distribute. .
Simplify. .
Collect the terms with c to the right. .
Factor the expression on the right. .
To isolate c , divide both sides by m 1 . .
Simplify by removing common factors. .

Notice that even though we excluded c = 0 and m = 0 from the original equation, we must also now state that m 1 .

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Solve: 1 a + 1 b = c for a .

a = b c b 1

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Solve: 2 x + 1 3 = 1 y for y .

y = 3 x 6 + x

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Key concepts

  • Strategy to Solve Equations with Rational Expressions
    1. Note any value of the variable that would make any denominator zero.
    2. Find the least common denominator of all denominators in the equation.
    3. Clear the fractions by multiplying both sides of the equation by the LCD.
    4. Solve the resulting equation.
    5. Check.
    • If any values found in Step 1 are algebraic solutions, discard them.
    • Check any remaining solutions in the original equation.

Practice makes perfect

Solve Rational Equations

In the following exercises, solve.

1 2 m = 8 m 2

−2 , 4

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1 + 9 p = −20 p 2

−5 , −4

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3 x + 4 + 7 x 4 = 8 x 2 16

−2

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5 y 9 + 1 y + 9 = 18 y 2 81

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8 z 10 + 7 z + 10 = 5 z 2 100

1 3

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9 a + 11 + 6 a 11 = 7 a 2 121

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1 q + 4 7 q 2 = 1

−2 , −1

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3 r + 10 4 r 4 = 1

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1 t + 7 5 t 5 = 1

−5 , −1

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2 s + 7 3 s 3 = 1

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v 10 v 2 5 v + 4 = 3 v 1 6 v 4

no solution

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w + 8 w 2 11 w + 28 = 5 w 7 + 2 w 4

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x 10 x 2 + 8 x + 12 = 3 x + 2 + 4 x + 6

no solution

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y 3 y 2 4 y 5 = 1 y + 1 + 8 y 5

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z 16 + z + 2 4 z = 1 2 z

−4

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b + 3 3 b + b 24 = 1 b

−8

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c + 3 12 c + c 36 = 1 4 c

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d d + 3 = 18 d 2 9 + 4

2

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m m + 5 = 50 m 2 25 + 6

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n n + 2 = 8 m 2 4 + 3

1

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p p + 7 = 98 p 2 49 + 8

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q 3 q 9 3 4 q + 12
= 7 q 2 + 6 q + 63 24 q 2 216

no solution

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r 3 r 15 1 4 r + 20
= 3 r 2 + 17 r + 40 12 r 2 300

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s 2 s + 6 2 5 s + 5
= 5 s 2 3 s 7 10 s 2 + 40 s + 30

no solution

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t 6 t 12 5 2 t + 10
= t 2 23 t + 70 12 t 2 + 36 t 120

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Solve a Rational Equation for a Specific Variable

In the following exercises, solve.

C r = 2 π for r

r = C 2 π

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V h = l w for h

h = v l w

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v + 3 w 1 = 1 2 for w

w = 2 v + 7

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x + 5 2 y = 4 3 for y

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a = b + 3 c 2 for c

c = b + 3 + 2 a a

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1 p + 2 q = 4 for p

p = q 4 q 2

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2 v + 1 5 = 1 2 for v

w = 15 v 10 + v

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m + 3 n 2 = 4 5 for n

n = 5 m + 23 n

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3 x 5 y = 1 4 for y

y = 20 x 12 x

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r = s 3 t for t

t = 3 r s r

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Everyday math

House Painting Alain can paint a house in 4 days. Spiro would take 7 days to paint the same house. Solve the equation 1 4 + 1 7 = 1 t for t to find the number of days it would take them to paint the house if they worked together.

2 6 11 days

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Boating Ari can drive his boat 18 miles with the current in the same amount of time it takes to drive 10 miles against the current. If the speed of the boat is 7 knots, solve the equation 18 7 + c = 10 7 c for c to find the speed of the current.

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Writing exercises

Why is there no solution to the equation 3 x 2 = 5 x 2 ?

Answers will vary.

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Pete thinks the equation y y + 6 = 72 y 2 36 + 4 has two solutions, y = −6 and y = 4 . Explain why Pete is wrong.

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Self check

After completing the exercises, use this checklist to evaluate your mastery of the objectives of this section.

This table has three rows and four columns. The first row is a header row and it labels each column. The first column is labeled

After reviewing this checklist, what will you do to become confident for all objectives?

Questions & Answers

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Source:  OpenStax, Elementary algebra. OpenStax CNX. Jan 18, 2017 Download for free at http://cnx.org/content/col12116/1.2
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