Login
📚 Intermediate Algebra 2e
Chapters ▾
⇩ Download ▾

7.6 Solve Rational Inequalities

Solve Rational Inequalities

We learned to solve linear inequalities after learning to solve linear equations. The techniques were very much the same with one major exception. When we multiplied or divided by a negative number, the inequality sign reversed.

Having just learned to solve rational equations we are now ready to solve rational inequalities. A rational inequality is an inequality that contains a rational expression.

Inequalities such as 32x>1,2xx3<4,2x3x6x, and 142x23x are rational inequalities as they each contain a rational expression.

When we solve a rational inequality, we will use many of the techniques we used solving linear inequalities. We especially must remember that when we multiply or divide by a negative number, the inequality sign must reverse.

Another difference is that we must carefully consider what value might make the rational expression undefined and so must be excluded.

When we solve an equation and the result is x=3, we know there is one solution, which is 3.

When we solve an inequality and the result is x>3, we know there are many solutions. We graph the result to better help show all the solutions, and we start with 3. Three becomes a zero partition number and then we decide whether to shade to the left or right of it. The numbers to the right of 3 are larger than 3, so we shade to the right.

This figure shows the solution, the interval 3 to infinity, of the inequality x is greater than 3 on a number line. The values range from negative 5 to 5 on the number line. The inequality is modeled by an open parenthesis at the zero partition number 3 and shading the right.

To solve a rational inequality, we first must write the inequality with only one quotient on the left and 0 on the right.

Next we determine the zero partition numbers to use to divide the number line into intervals. A zero partition number is a number which make the rational expression zero or undefined.

We then will evaluate the factors of the numerator and denominator, and find the quotient in each interval. This will identify the interval, or intervals, that contains all the solutions of the rational inequality.

We write the solution in interval notation being careful to determine whether the endpoints are included.

We summarize the steps for easy reference.

The next example requires that we first get the rational inequality into the correct form.

In the next example, the numerator is always positive, so the sign of the rational expression depends on the sign of the denominator.

The next example requires some work to get it into the needed form.

Solve an Inequality with Rational Functions

When working with rational functions, it is sometimes useful to know when the function is greater than or less than a particular value. This leads to a rational inequality.

In economics, the function C(x) is used to represent the cost of producing x units of a commodity. The average cost per unit can be found by dividing C(x) by the number of items x. Then, the average cost per unit is c(x)=C(x)x.

Key Concepts

  • Solve a rational inequality.
    1. Write the inequality as one quotient on the left and zero on the right.
    2. Determine the zero partition numbers–the points where the rational expression will be zero or undefined.
    3. Use the zero partition numbers to divide the number line into intervals.
    4. Test a value in each interval. Above the number line show the sign of each factor of the rational expression in each interval. Below the number line show the sign of the quotient.
    5. Determine the intervals where the inequality is correct. Write the solution in interval notation.

Section Exercises

Practice Makes Perfect

Solve Rational Inequalities

In the following exercises, solve each rational inequality and write the solution in interval notation.

x3x+40

(,−4)[3,)

x+6x50

x+1x30

[−1,3)

x4x+20

x7x1>0

(,1)(7,)

x+8x+3>0

x6x+5<0

(−5,6)

x+5x2<0

3xx5<1

(52,5)

5xx2<1

6xx6>2

(,−3)(6,)

3xx4>2

2x+3x61

[−9,6)

4x1x41

3x2x42

(,−6](4,)

4x3x32

1x2+7x+12>0

(,−4)(−3,)

1x24x12>0

3x25x+4<0

(1,4)

4x2+7x+12<0

22x2+x150

(,−3)(52,)

63x22x50

−26x213x+60

(,23)(32,)

−110x2+11x60

12+12x2>5x

(,0)(0,4)(6,)

13+1x2>43x

124x21x

[−2,0)(0,4]

1232x21x

1x216<0

(−4,4)

4x225>0

4x23x+1

[−10,−1)(2,)

5x14x+2

Solve an Inequality with Rational Functions

In the following exercises, solve each rational function inequality and write the solution in interval notation.

Given the function R(x)=x5x2, find the values of x that make the function less than or equal to 0.

(2,5]

Given the function R(x)=x+1x+3, find the values of x that make the function greater than or equal to 0.

Given the function R(x)=x6x+2, find the values of x that make the function less than or equal to 0.

(−2,6]

Given the function R(x)=x+1x4, find the values of x that make the function less than or equal to 0.

Writing Exercises

Write the steps you would use to explain solving rational inequalities to your little brother.

Answers will vary.

Create a rational inequality whose solution is (,−2][4,).

Self Check

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

This table has four columns and three rows. The first row is a header and it labels each column, “I can…”, “Confidently,” “With some help,” and “No-I don’t get it!” In row 2, the I can was solve rational inequalities. In row 3, the I can was solve an inequality with rational functions.

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

Chapter Review Exercises

Simplify, Multiply, and Divide Rational Expressions

Determine the Values for Which a Rational Expression is Undefined

In the following exercises, determine the values for which the rational expression is undefined.

5a+33a2

a23

b7b225

5x2y28y

y0

x3x2x30

Simplify Rational Expressions

In the following exercises, simplify.

1824

34

9m418mn3

x2+7x+12x2+8x+16

x+3x+4

7v3525v2

Multiply Rational Expressions

In the following exercises, multiply.

58·415

16

3xy28y3·16y224x

72x12x28x+32·x2+10x+24x236

−3x2

2y2+y34y2·y24y+42y2+11y+12

Divide Rational Expressions

In the following exercises, divide.

x24x12x2+8x+12÷x2363x

3x(x+6)(x+6)

y2164÷y3642y2+8y+32

11+ww9÷121w29w

111w

3y212y634y+3÷(6y242y)

c2643c2+26c+16c24c3215c+10

5c+4

8a2+16aa4·a2+2a24a2+7a+10÷2a26aa+5

Multiply and Divide Rational Functions

Find R(x)=f(x)·g(x) where f(x)=9x2+9xx23x4 and g(x)=x2163x2+12x.

R(x)=3

Find R(x)=f(x)g(x) where f(x)=27x23x21 and
g(x)=9x2+54xx2x42.

Add and Subtract Rational Expressions

Add and Subtract Rational Expressions with a Common Denominator

In the following exercises, perform the indicated operations.

715+815

1

4a22a112a1

y2+10yy+5+25y+5

y+5

7x2x29+21xx29

x2x73x+28x7

x+4

y2y+11121y+11

4q2q+3q2+6q+53q2+q+6q2+6q+5

q3q+5

5t2+4t2+3t2254t28t32t225

Add and Subtract Rational Expressions Whose Denominators Are Opposites

In the following exercises, add and subtract.

18w6w1+3w216w

15w+26w1

a2+3aa243a+84a2

2b2+3b15b249b2+16b149b2

3b2+19b16b249

8y210y+72y5+2y2+7y+252y

Find the Least Common Denominator of Rational Expressions

In the following exercises, find the LCD.

7a23a10,3aa2a20

(a+2)(a5)(a+4)

6n24,2nn24n+4

53p2+17p6,2m3p2+23p8

(3p1)(p+6)(p+8)

Add and Subtract Rational Expressions with Unlike Denominators

In the following exercises, perform the indicated operations.

75a+32b

2c2+9c+3

11c12(c2)(c+3)

3xx29+5x2+6x+9

2xx2+10x+24+3xx2+8x+16

5x2+26x(x+4)(x+4)(x+6)

5qp2qp2+4qq21

3yy+2y+2y+8

2(y2+10y2)(y+2)(y+8)

−3w15w2+w20w+24w

7m+3m+25

2m7m+2

nn+3+2n3n+9n29

8aa2644a+8

4a8

512x2y+720xy3

Add and Subtract Rational Functions

In the following exercises, find R(x)=f(x)+g(x) where f(x) and g(x) are given.

f(x)=2x2+12x11x2+3x10,g(x)=x+12x

R(x)=x+8x+5

f(x)=−4x+31x2+x30,g(x)=5x+6

In the following exercises, find R(x)=f(x)g(x) where f(x) and g(x) are given.

f(x)=4xx2121,g(x)=2x11

R(x)=2x+11

f(x)=7x+6,g(x)=14xx236

Simplify Complex Rational Expressions

Simplify a Complex Rational Expression by Writing It as Division

In the following exercises, simplify.

7xx+214x2x24

x22x

25+5613+14

x3xx+51x+5+1x5

(x+2)(x5)2

2m+mnnm1n

Simplify a Complex Rational Expression by Using the LCD

In the following exercises, simplify.

13+1814+112

118

3a21b1a+1b2

2z249+1z+79z+7+12z7

z521z+21

3y24y322y8+1y+4

7.4 Solve Rational Equations

Solve Rational Equations

In the following exercises, solve.

12+23=1x

x=67

12m=8m2

1b2+1b+2=3b24

b=32

3q+82q2=1

v15v29v+18=4v3+2v6

no solution

z12+z+33z=1z

Solve Rational Equations that Involve Functions

For rational function, f(x)=x+2x26x+8, ⓐ find the domain of the function ⓑ solve f(x)=1 ⓒ find the points on the graph at this function value.

ⓐ The domain is all real numbers except x2 and x4.x=1,x=6
(1,1),(6,1)

For rational function, f(x)=2xx2+7x+10, ⓐ find the domain of the function ⓑ solve f(x)=2 ⓒ find the points on the graph at this function value.

Solve a Rational Equation for a Specific Variable

In the following exercises, solve for the indicated variable.

Vl=hw for l.

l=Vhw

1x2y=5 for y.

x=y+5z7 for z.

z=y+5+7xx

P=kV for V.

Solve Applications with Rational Equations

Solve Proportions

In the following exercises, solve.

x4=35

x=125

3y=95

ss+20=37

s=15

t35=t+29

Solve Using Proportions

In the following exercises, solve.

Rachael had a 21-ounce strawberry shake that has 739 calories. How many calories are there in a 32-ounce shake?

1126 calories

Leo went to Mexico over Christmas break and changed $525 dollars into Mexican pesos. At that time, the exchange rate had $1 US is equal to 16.25 Mexican pesos. How many Mexican pesos did he get for his trip?

Solve Similar Figure Applications

In the following exercises, solve.

ΔABC is similar to ΔXYZ. The lengths of two sides of each triangle are given in the figure. Find the lengths of the third sides.

The first figure is triangle A B C with side A B 8 units long, side B C 7 units long, and side A C b units long. The second figure is triangle X Y Z with side X Y 2 and two-thirds units long, side Y Z x units long, and side X Z 3 units long.

b=9;x=213

On a map of Europe, Paris, Rome, and Vienna form a triangle whose sides are shown in the figure below. If the actual distance from Rome to Vienna is 700 miles, find the distance from
ⓐ Paris to Rome
ⓑ Paris to Vienna

The figure is a triangle formed by Paris, Vienna, and Rome. The distance between Paris and Vienna is 7.7 centimeters. The distance between Vienna and Rome is 7 centimeters. The distance between Rome and Paris is 8.9 centimeters.

Francesca is 5.75 feet tall. Late one afternoon, her shadow was 8 feet long. At the same time, the shadow of a nearby tree was 32 feet long. Find the height of the tree.

23 feet

The height of a lighthouse in Pensacola, Florida is 150 feet. Standing next to the statue, 5.5-foot-tall Natasha cast a 1.1-foot shadow. How long would the shadow of the lighthouse be?

Solve Uniform Motion Applications

In the following exercises, solve.

When making the 5-hour drive home from visiting her parents, Lolo ran into bad weather. She was able to drive 176 miles while the weather was good, but then driving 10 mph slower, went 81 miles when it turned bad. How fast did she drive when the weather was bad?

45 mph

Mark is riding on a plane that can fly 490 miles with a headwind of 20 mph in the same time that it can fly 350 miles against a tailwind of 20 mph. What is the speed of the plane?

Josue can ride his bicycle 8 mph faster than Arjun can ride his bike. It takes Arjun 3 hours longer than Josue to ride 48 miles. How fast can Josue ride his bike?

16 mph

Curtis was training for a triathlon. He ran 8 kilometers and biked 32 kilometers in a total of 3 hours. His running speed was 8 kilometers per hour less than his biking speed. What was his running speed?

Solve Work Applications

In the following exercises, solve.

Brandy can frame a room in 1 hour, while Jake takes 4 hours. How long could they frame a room working together?

48 minutes

Prem takes 3 hours to mow the lawn while her cousin, Barb, takes 2 hours. How long will it take them working together?

Jeffrey can paint a house in 6 days, but if he gets a helper he can do it in 4 days. How long would it take the helper to paint the house alone?

12 days

Marta and Deb work together writing a book that takes them 90 days. If Marta worked alone it would take her 120 days. How long would it take Deb to write the book alone?

Solve Direct Variation Problems

In the following exercises, solve.

If y varies directly as x when y=9 and x=3, find x when y=21.

x=7

If y varies inversely as x when y=20 and x=2, find y when x=4.

Vanessa is traveling to see her fiancé. The distance, d, varies directly with the speed, v, she drives. If she travels 258 miles driving 60 mph, how far would she travel going 70 mph?

301 mph

If the cost of a pizza varies directly with its diameter, and if an 8” diameter pizza costs $12, how much would a 6” diameter pizza cost?

The distance to stop a car varies directly with the square of its speed. It takes 200 feet to stop a car going 50 mph. How many feet would it take to stop a car going 60 mph?

288 feet

Solve Inverse Variation Problems

In the following exercises, solve.

If m varies inversely with the square of n, when m=4 and n=6 find m when n=2.

The number of tickets for a music fundraiser varies inversely with the price of the tickets. If Madelyn has just enough money to purchase 12 tickets for $6 each, how many tickets can Madelyn afford to buy if the price increased to $8?

9 tickets

On a string instrument, the length of a string varies inversely with the frequency of its vibrations. If an 11-inch string on a violin has a frequency of 360 cycles per second, what frequency does a 12-inch string have?

Solve Rational Inequalities

Solve Rational Inequalities

In the following exercises, solve each rational inequality and write the solution in interval notation.

x3x+40

(−4,3]

5xx2>1

3x2x42

[−6,4)

1x24x12<0

124x21x

(,−2][4,)

4x2<3x+1

Solve an Inequality with Rational Functions

In the following exercises, solve each rational function inequality and write the solution in interval notation

Given the function, R(x)=x5x2, find the values of x that make the function greater than or equal to 0.

(,2)[5,)

Given the function, R(x)=x+1x+3, find the values of x that make the function less than or equal to 0.

The function
C(x)=150x+100,000 represents the cost to produce x, number of items. Find ⓐ the average cost function, c(x) ⓑ how many items should be produced so that the average cost is less than $160.

c(x)=150x+100000x
ⓑ More than 10,000 items must be produced to keep the average cost below $160 per item.

Tillman is starting his own business by selling tacos at the beach. Accounting for the cost of his food truck and ingredients for the tacos, the function C(x)=2x+6,000 represents the cost for Tillman to produce x, tacos. Find ⓐ the average cost function, c(x) for Tillman’s Tacos ⓑ how many tacos should Tillman produce so that the average cost is less than $4.

Practice Test

In the following exercises, simplify.

4a2b12ab2

a3b

6x18x29

In the following exercises, perform the indicated operation and simplify.

4xx+2·x2+5x+612x2

x+33x

2y2y21÷y3y2+yy3+1

6x2x+20x2815x2+11x7x281

x3x+9

−3a3a3+5aa2+3a4

2n2+8n1n21n27n11n2

3n2n1

10x2+16x78x3+2x2+3x138x

1m1n1n+1m

nmm+n

In the following exercises, solve each equation.

1x+34=58

1z5+1z+5=1z225

z=12

z2z+834z8=3z216z168z2+16z64

In the following exercises, solve each rational inequality and write the solution in interval notation.

6xx62

[−3,6)

2x+3x6>1

12+12x25x

(,0)(0,4][6,)

In the following exercises, find R(x) given f(x)=x4x23x10 and g(x)=x5x22x8.

R(x)=f(x)g(x)

R(x)=f(x)·g(x)

R(x)=1(x+2)(x+2)

R(x)=f(x)÷g(x)

Given the function,
R(x)=22x2+x15, find the values of x that make the function less than or equal to 0.

(−3,52)

In the following exercises, solve.

If y varies directly with x, and x=5 when y=30, find x when y=42.

If y varies inversely with the square of x and x=3 when y=9, find y when x=4.

y=8116

Matheus can ride his bike for 30 miles with the wind in the same amount of time that he can go 21 miles against the wind. If the wind’s speed is 6 mph, what is Matheus’ speed on his bike?

Oliver can split a truckload of logs in 8 hours, but working with his dad they can get it done in 3 hours. How long would it take Oliver’s dad working alone to split the logs?

Oliver’s dad would take 445 hours to split the logs himself.

The volume of a gas in a container varies inversely with the pressure on the gas. If a container of nitrogen has a volume of 29.5 liters with 2000 psi, what is the volume if the tank has a 14.7 psi rating? Round to the nearest whole number.

The cities of Dayton, Columbus, and Cincinnati form a triangle in southern Ohio. The diagram gives the map distances between these cities in inches.

The figure is a triangle formed by Cincinnati, Dayton, and Columbus. The distance between Cincinnati and Dayton is 2.4 inches. The distance between Dayton and Columbus is 3.2 inches. The distance between Columbus and Cincinnati is 5.3 inches.

The actual distance from Dayton to Cincinnati is 48 miles. What is the actual distance between Dayton and Columbus?

The distance between Dayton and Columbus is 64 miles.