The Purplemath Forums

by **little_dragon** on Mon Dec 08, 2008 5:43 pm

I know some about powers and about solving by factoring. One of my "challenge" problems is to solve 2^(2x) - 7(2^x) - 8 = 0, and the hint in the book says "Let y = 2^x". I think this gives me y^2 - 7y - 8 = 0. Is that right? Because then I'm pretty sure I can finish this. Thanks.

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by **stapel_eliz** on Mon Dec 08, 2008 6:29 pm

little_dragon wrote:I think this gives me y^2 - 7y - 8 = 0. Is that right?

That's exactly right! :wink:

Now factor, and set each factor equal to zero. Once you've done that, plug the $2^x$ back in for the y, and use what you know about powers to find the answers.

If you get stuck, please reply showing how far you got. Thank you!

Eliz.

The Purplemath Forums

Solve 2^(2x) - 7(2^x) - 8 = 0 (Hint: Let y = 2^x)

Solve 2^(2x) - 7(2^x) - 8 = 0 (Hint: Let y = 2^x)