Download Solving Equations

13
4C. SOLVING EQUATIONS
4c
Solving Equations
We can add or subtract the same decimal number from both sides of an equation
without affecting the solution, just as we do with fractions. In fact, all of the
techniques for solving equations that we have learned work with decimal values.
You Try It!
EXAMPLE 1. Solve for x:
x − 1.35 = −2.6.
Solution. To undo subtracting 1.35, add 1.35 to both sides of the equation.
x − 1.35 = −2.6
Solve for x:
x + 1.25 = 0.6
Original equation.
x − 1.35 + 1.35 = −2.6 + 1.35
x = −1.25
Add 1.35 to both sides.
Simplify: −2.6 + 1.35 = −1.25.
Answer: −0.65
!
You Try It!
EXAMPLE 2. Solve for x:
−1.2x = −4.08.
Solution. To undo multiplying by −1.2, divide both sides of the equation by
−1.2.
−1.2x = −4.08
−1.2x
−4.08
=
−1.2
−1.2
x = 3.4
Solve for z:
−2.5z = 1.4
Original equation.
Divide both sides by −1.2.
Simplify: −4.08/(−1.2) = 3.4.
Answer: −0.56
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Combining Operations
Sometimes, more than one operation is needed to solve a given equation.
You Try It!
EXAMPLE 3. Solve for x:
−3.8x − 1.7 = −17.28.
Solution. To undo subtracting 1.7, add 1.7 to both sides of the equation.
−3.8x − 1.7 = −17.28
−3.8x − 1.7 + 1.7 = −17.28 + 1.7
−3.8x = −15.58
Original equation.
Add 1.7 to both sides
Simplify: −17.28 + 1.7 = −15.58.
Next, to undo multiplying by −3.8, divide both sides of the equation by −3.8.
Solve for u:
−0.02u − 3.2 = −1.75
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MODULE 4. DECIMALS
−15.58
−3.8x
=
−3.8
−3.8
x = 4.1
Divide both sides by −3.8.
Simplify: −15.58/(−3.8) = 4.1.
Answer: −72.5
Combining Like Terms
Combining like terms with decimal coefficients is done in the same manner as
combining like terms with integer coefficients.
You Try It!
Solve for r:
−4.2 + 3.6r − 4.1r = 1.86
EXAMPLE 4. Solve the equation for x: 4.2 − 3.1x + 2x = −7.02.
Solution. Combine like terms on the left-hand side of the equation.
4.2 − 3.1x + 2x = −7.02
4.2 − 1.1x = −7.02
4.2 − 1.1x − 4.2 = −7.02 − 4.2
−1.1x = −11.02
−1.1x
−11.22
=
−1.1
−1.1
x = 10.2
Original equation.
Combine like terms: −3.1x + 2x = −1.1x.
Subtract 4.2 from both sides.
Subtract: −7.02 − 4.2 = −11.22.
Divide both sides by −1.1.
Divide: −11.22/(−1.1) = 10.2.
Thus, the solution of the equation is 10.2.
Check. As with all equations, we check our solution by substituting our answer
into the original equation.
4.2 − 3.1x + 2x = −7.02 Original equation.
4.2 − 3.1(10.2) + 2(10.2) = −7.02 Substitute 10.2 for x.
4.2 − 31.62 + 20.4 = −7.02 Multiply: 3.1(10.2) = 31.62, 2(10.2) = 20.4.
−27.42 + 20.4 = −7.02 Order of Ops: Add, left to right.
4.2 − 31.62 = −27.42.
−7.02 = −7.02 Add: −27.42 + 20.4 = −7.02.
Answer: −12.12
Because the last line is a true statement, the solution x = 10.2 checks.
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15
4C. SOLVING EQUATIONS
Using the Distributive Property
Sometimes the distributive property is needed when solving equations.
Distributive Property. Let a, b, and c be any numbers. Then,
a(b + c) = ab + ac.
You Try It!
EXAMPLE 5. Solve the equation for x: −6.3x − 0.4(x − 1.2) = −0.86.
Solution. We first distribute the −0.4 times each term in the parentheses,
then combine like terms.
−6.3x − 0.4(x − 1.2) = −0.86
−6.3x − 0.4x + 0.48 = −0.86
−6.7x + 0.48 = −0.86
Solve for x:
−2.5x − 0.1(x − 2.3) = 8.03
Original equation.
Distribute. Note that −0.4(−1.2) = 0.48.
Combine like terms.
Next, subtract 0.48 from both sides, then divide both sides of the resulting
equation by −6.7.
−6.7x + 0.48 − 0.48 = −0.86 − 0.48
−6.7x = −1.34
−1.34
−6.7x
=
−6.7
−6.7
x = 0.2
Subtract 0.48 from both sides.
Simplify: −0.86 − 0.48 = −1.34.
Divide both sides by −6.7.
Simplify: −1.34/(−6.7) = 0.2.
Answer: −3
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Rounding Solutions
An approximate solution is sometimes adequate.
You Try It!
EXAMPLE 6. Solve the equation
answer to the nearest tenth.
3.1x + 4.6 = 2.5 − 2.2x for x. Round the
Solution. We need to isolate the terms containing x on one side of the equation. To isolate the the terms containing x to left hand side of the equation,
Solve for x:
4.2x − 1.25 = 3.4 + 0.71x
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MODULE 4. DECIMALS
add 2.2x to both sides.
Original equation.
3.1x + 4.6 = 2.5 − 2.2x
3.1x + 4.6 + 2.2x = 2.5 − 2.2x + 2.2x Add 2.2x to both sides.
5.3x + 4.6 = 2.5
Combine terms: 3.1x + 2.2x = 5.3x.
To undo adding 4.6, subtract 4.6 from both sides of the equation.
5.3x + 4.6 − 4.6 = 2.5 − 4.6
5.3x = −2.1
Subtract 4.6 from both sides.
Simplify: 2.5 − 4.6 = −2.1.
To further isolate x, divide both sides of the equation by 5.3.
5.3x
−2.1
=
5.3
5.3
x ≈ −0.4
To round the answer to the nearest tenth, we must carry the division out one additional place.
0.39
53)21.00
15 9
5 10
4 77
33
Answer: 1.33
Divide both sides by 5.3.
Round solution to nearest tenth.
Because the “test digit” is greater
than or equal to 5, add 1 to the
rounding digit and truncate.
Test digit
−0. 3 9
Rounding digit
Thus, −0.39 ≈ −0.4.
Thus, −2.1/5.3 ≈ −0.39.
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