Big Ideas Math Algebra 2, 2014
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Big Ideas Math Algebra 2, 2014 View details
6. Solving Exponential and Logarithmic Equations
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Exercise 69 Page 340

If two equivalent logarithmic expressions have the same base, then the arguments must be equal.

x ≈ 10.61

Practice makes perfect
We want to solve an equation involving more than one logarithm. To do so, we will use the Change of Base Formula to evaluate the given expression. log_b m = log_c m/log_c b Numbers m, b, and c are positive numbers, with b≠ 1 and c≠ 1. We will apply this formula with c=9 to simplify one of the expressions.
log_3(x-6) = log_9 2x

log_c a = log_b a/log_b c

log_9 (x-6)/log_9 3 = log_9 2x

Calculate logarithm

log_9 (x-6)/1/2 = log_9 2x
log_9(x-6) * 2 = log_9 2x
2 log_9(x-6) = log_9 2x

m* log_9(a)=log_9(a^m)

log_9(x-6)^2 = log_9 2x
Next, we will use the fact that if two equivalent logarithmic expressions have the same base, then the arguments must be equal. log_b x=log_b y ⇔ x= y Let's apply the above property to our equation.
log_9(x-6)^2 = log_9 2x

Equate arguments

(x-6)^2=2x
x^2-12x+36=2x
x^2-14x+36=0
We will use the Quadratic Formula to solve the given quadratic equation. ax^2+ bx+ c=0 ⇕ x=- b± sqrt(b^2-4 a c)/2 a We first need to identify the values of a, b, and c. x^2-14x+36=0 ⇕ 1x^2+( -14)x+ 36=0 We see that a= 1, b= -14, and c= 36. Let's substitute these values into the Quadratic Formula.
x=- b±sqrt(b^2-4ac)/2a
x=- ( -14)±sqrt(( -14)^2-4( 1)( 36))/2( 1)
Evaluate right-hand side
x=14±sqrt((-14)^2-4(1)(36))/2(1)
x=14±sqrt(196-4(1)(36))/2(1)
x=14±sqrt(196-4(36))/2
x=14±sqrt(196-144)/2
x=14±sqrt(52)/2
x=14±sqrt(4(13))/2
x = 14± sqrt(4)sqrt(13)/2
x = 14± 2sqrt(13)/2
x = 2(7± sqrt(13))/2
x = 7± sqrt(13)
Using the Quadratic Formula, we found that the solutions of the given equation are x=7± sqrt(13). Therefore, the exact solutions are x_1=7+ sqrt(13) and x_2=7-sqrt(13). Let's use a calculator to approximate these values. x_1 & = 7+ sqrt(13) ≈ 10.61 x_2 & = 7-sqrt(13) ≈ 3.39 To check our answer, we will substitute both 10.61 and 3.39 for x in the given equation one at a time.
log_3(x-6) = log_9 2x
log_3( 10.61-6) ? = log_9 2( 10.61)
log_3 4.61 ? = log_9 2(10.61)
log_3 4.61 ? = log_9 21.22

Calculate logarithm

1.39 = 1.39 ✓
Since substituting 10.61 for x in the given equation produces a true statement, x=10.61 is the solution to our equation. Let's now check for the x=3.39.
log_3(x-6) = log_9 2x
log_3( 3.39-6) ? = log_9 2( 3.39)
log_3(- 2.61) ? = log_9 2(3.39)
log_3 (- 2.61) ? = log_9 6.78 *
The argument of a logarithmic function cannot be negative, so log_3 (- 2.6) is undefined, and 3.39 is an extraneous solution.