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§ 6.6
Rational Equations
Blitzer, Intermediate Algebra, 4e – Slide #87
Solving a Rational Equation
EXAMPLEEXAMPLE
Solve:
SOLUTIONSOLUTION
Notice that the variable x appears in two of the denominators. We must avoid any values of the variable that make a denominator zero.
.4
5
9
5
47
xx
x
This denominator would equal zero if x = 0.
xx
x 4
5
9
5
47
This denominator would equal zero if x = 0.
Therefore, we see that x cannot equal zero.
Blitzer, Intermediate Algebra, 4e – Slide #88
Solving a Rational Equation
The denominators are 5x, 5, and x. The least common denominator is 5x. We begin by multiplying both sides of the equation by 5x. We will also write the restriction that x cannot equal zero to the right of the equation.
0,4
5
9
5
47
x
xx
x
CONTINUECONTINUEDD
This is the given equation.
xx
x
xx
4
5
95
5
475 Multiply both sides by 5x,
the LCD.
x
xx
x
xx 4
1
5
5
9
1
5
5
47
1
5
Use the distributive property.
Blitzer, Intermediate Algebra, 4e – Slide #89
Solving a Rational Equation
CONTINUECONTINUEDD
45947 xx Divide out common factors in the multiplications.
20947 xx Multiply.
2024 x Subtract.
x216 Add.
x8 Divide.
The proposed solution, 8, is not part of the restriction . It should check in the original equation.
0x
Blitzer, Intermediate Algebra, 4e – Slide #90
Solving a Rational Equation
CONTINUECONTINUEDD Check 8:
xx
x 4
5
9
5
47
8
4
5
9
85
487
8
4
5
9
40
456
8
4
5
9
40
52
8
4
5
940
40
5240
8
440
5
94052
459852
207252
5252
This true statement verifies that the solution is 8 and the solution set is {8}.
?
?
?
?
?
?
?
true
Blitzer, Intermediate Algebra, 4e – Slide #91
Solving a Rational Equation
Solving Rational Equations1) List restrictions on the variable. Avoid any values of the variable that make a denominator zero.
2) Clear the equation of fractions by multiplying both sides by the LCD of all rational expressions in the equation.
3) Solve the resulting equation.
4) Reject any proposed solution that is in the list of restrictions on the variable. Check other proposed solutions in the original equation.
Blitzer, Intermediate Algebra, 4e – Slide #92
Solving a Rational Equation
EXAMPLEEXAMPLE
Solve:
SOLUTIONSOLUTION
1) List restrictions on the variable.
.72
56
4
13
x
x
x
x
This denominator would equal zero if x = 4.
This denominator would equal zero if x = 3.5.
The restrictions are
72
56
4
13
x
x
x
x
.5.3 and 4 xx
Blitzer, Intermediate Algebra, 4e – Slide #93
Solving a Rational Equation
2) Multiply both sides by the LCD. The denominators are x – 4 and 2x – 7. Thus, the LCD is (x – 4)(2x + 7).
5.3 ,4 ,72
56
4
13
xxx
x
x
x
CONTINUECONTINUEDD
This is the given equation.
72
56724
4
13724
x
xxx
x
xxx Multiply both sides by
the LCD.
Simplify. 5641372 xxxx
Blitzer, Intermediate Algebra, 4e – Slide #94
Solving a Rational Equation
4) Check the proposed solution in the original equation. Notice, there is no proposed solution. And of course, -7 = -20 is not a true statement. Therefore, there is no solution to the original rational equation. We say the solution set is , the empty set.
CONTINUECONTINUEDD
This is the equation cleared of fractions.
5641372 xxxx
Use FOIL on each side.201967196 22 xxxx
Subtract from both sides.2019719 xx
207
26x
Subtract 19x from both sides.
3) Solve the resulting equation.
Blitzer, Intermediate Algebra, 4e – Slide #95
Solving a Rational Equation
EXAMPLEEXAMPLE
Solve:
SOLUTIONSOLUTION
1) List restrictions on the variable. By factoring denominators, it makes it easier to see values that make the denominators zero.
.2
1
4
2
82
122
xxxx
x
This denominator is zero if x = -4 or x = 2.
This denominator would equal zero if x = -4.
The restrictions are .2 and 4 xx
2
1
4
2
24
12
xxxx
x
This denominator would equal zero if x = 2.
Blitzer, Intermediate Algebra, 4e – Slide #96
Solving a Rational Equation
2) Multiply both sides by the LCD. The factors of the LCD are x + 4 and x – 2. Thus, the LCD is (x + 4)(x - 2).
CONTINUECONTINUEDD
This is the given equation.
Multiply both sides by the LCD.
Use the distributive property.
2 ,4 ,2
1
4
2
24
12
xxxxxx
x
2
124
4
2
24
1224
xxx
xxx
xxx
2
124
4
224
24
1224
xxx
xxx
xx
xxx
Blitzer, Intermediate Algebra, 4e – Slide #97
Solving a Rational Equation
3) Solve the resulting equation.
CONTINUECONTINUEDD
Simplify. 42212 xxx
42212 xxx This is the equation with cleared fractions.
44212 xxx Use the distributive property.
454 xx Combine like terms.
453 x Subtract x from both sides.
93 x Add 5 to both sides.
3x Divide both sides by 3.
Blitzer, Intermediate Algebra, 4e – Slide #98
Solving a Rational Equation
4) Check the proposed solutions in the original equation. The proposed solution, 3, is not part of the restriction that Substitute 3 for x, in the given (original) equation. The resulting true statement verifies that 3 is a solution and that {3} is the solution set.
CONTINUECONTINUEDD
and 4x.2x
Solving a Rational Equation
EXAMPLEEXAMPLE
Rational functions can be used to model learning. Many of these functions model the proportion of correct responses as a function of the number of trials of a particular task. One such model, called a learning curve, is
where f (x) is the proportion of correct responses after x trials. If f (x) = 0, there are no correct responses. If f (x) = 1, all responses are correct. The graph of the rational function is shown on the next page. Use the function to solve the following problem.
1.09.0
4.09.0
x
xxf
Blitzer, Intermediate Algebra, 4e – Slide #99
Solving a Rational Equation
Blitzer, Intermediate Algebra, 4e – Slide #100
CONTINUECONTINUEDD
A Learning Curve
00.10.20.30.40.50.60.70.80.9
11.11.2
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Number of Learning Trials
Pro
po
rtio
n o
f C
orr
ec
t R
es
po
ns
es
1.09.0
4.09.0
x
xxf
Solving a Rational Equation
Blitzer, Intermediate Algebra, 4e – Slide #101
CONTINUECONTINUEDD
SOLUTIONSOLUTION
How many learning trials are necessary for 0.5 of the responses to be correct? Identify your solution as a point on the graph.
Substitute 0.5, the proportion of correct responses, for f (x) and solve the resulting rational equation for x.
1.09.0
4.09.05.0
x
xThe LCD is 0.9x + 0.1.
1.09.0
4.09.01.09.05.01.09.0
x
xxx Multiply both sides by the
LCD.
4.09.01.09.05.0 xx Simplify.
Solving a Rational Equation
Blitzer, Intermediate Algebra, 4e – Slide #102
CONTINUECONTINUEDD
4.09.005.045.0 xx Use the distributive property on the left side.
4.005.045.0 x Subtract 0.9x from both sides.
45.045.0 x Subtract 0.05 from both sides.
1x Divide both sides by -0.45.
The number of learning trials necessary for 0.5 of the responses to be correct is 1.
The solution is identified as a point on the graph at the beginning of the problem.