13
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Evaluating Logarithmic Expressions

Question 13 • FL-1200330-Algebra 2 CR

Which equation(s) have x = –3 as the solution?

Answer
A
log3(2x + 15) = 2
B
log5(8x + 9) = 2
C
log4(–20x + 4) = 3
D
logx81 = 4

Explanation

Substitute x = −3 into each equation. In A, 2(−3) + 15 = 9, and log base 3 of 9 is 2, so A works. In C, −20(−3) + 4 = 64, and log base 4 of 64 is 3, so C works. B gives a negative logarithm argument (−15), and D has a negative base, which is not valid for a real logarithm.

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