Wednesday
May 22, 2013

# Homework Help: math

Posted by sam on Wednesday, July 14, 2010 at 12:35am.

An exponential function "f" is defined by f(x)=7(1.5)^x-6. if f(k)=91, then what is the value of k?

• math - Reiny, Wednesday, July 14, 2010 at 6:32am

you are solving

7(1.5)^(k-6) = 91
1.5^(k-6) = 13

take log of both sides
log [1.5^(k-6)] = log 13
(k-6)[log 1.5] = log 13
k-6 = log13/log1.5 = 6.3259
k = 12.3259

check
7(1.5^6.3259)
= 7(12.99977) = 90.9984 (not bad)

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