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December 20, 2014

December 20, 2014

Posted by **Rachal** on Tuesday, February 15, 2011 at 11:09pm.

log3(x+7)=2-log3(x-1)

Write the exact answer using base-10 logarithms.

- Math -
**Reiny**, Tuesday, February 15, 2011 at 11:21pmlog3(x+7)=2-log3(x-1)

log3(x+7) + log3(x-1) = 2

log3 [(x+7)/(x-1)] = 2

(x+7)(x-1) = 3^2

x^2 + 6x - 7 = 9

x^2 + 6x - 16 = 0

(x+8)(x-2) = 0

x = -8 or x = 2

but x=-8 would make log3(x+7) undefined, so

x = 2

( why would it ask to use base 10 logs ????)

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