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

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Question 332264: Solve: 2^2x - 3*2^x - 40 = 0
Answer by jsmallt9(3758) About Me  (Show Source):
You can put this solution on YOUR website!
2%5E%282x%29+-+3%2A2%5Ex+-+40+=+0
The key to solving this is to notice that the exponent of 2 in the first term is twice the exponent of the exponent of 2 in the middle term. That makes this equation a quadratic equation in 2%5Ex. If you have trouble seeing this, let's use a temporary variable. Let q+=+2%5Ex, then q%5E2+=+%282%5Ex%29%5E2+=+2%5E%282x%29 Substituting into the original equation we get:
q%5E2+-3q+-+40+=+0
The quadratic nature of the equation should now be clear. We can solve for q by factoring (or using the Quadratic Formula):
%28q+-+8%29%28q+%2B5%29+=+0
Using the Zero Product Property:
q+-+8+=+0 or q+%2B+5+=+0
Solving we get:
q+=+8 or q+=+-5
Now we can substitute back for the temporary variable:
2%5Ex+=+8 or 2%5Ex+=+-5
From the first equation we can see that x must be 3. And since 2 to any power can never be negative, there is no solution for 2%5Ex+=+-5.

So the only solution to
2%5E%282x%29+-+3%2A2%5Ex+-+40+=+0
is
x = 3

With some practice you will no longer need a temporary variable. You will be able to go straight from
2%5E%282x%29+-+3%2A2%5Ex+-+40+=+0
to
%282%5Ex+-+8%29%282%5Ex+%2B+5%29+=+0