SOLUTION: Solve: 2^2x - 3 * 2^x - 40 = 0

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Question 481308: Solve: 2^2x - 3 * 2^x - 40 = 0
Answer by lwsshak3(11628) About Me  (Show Source):
You can put this solution on YOUR website!
Solve: 2^2x - 3 * 2^x - 40 = 0
**
let u=2^x
u^2=2^2x
..
u^2-3u-40=0
(u-8)(u+5)=0
u=8
or
u=-5
..
u=2^x=-5 (reject, 2^x > 0)
u=2^x=8=2^3
x=3