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Annual Harvard-MIT November Tournament

Algebra Difficulty 4.2 AIME Find the answer United States

Problem:
Find the product of all real xx for which
23x+1−17⋅22x+2x+3=0 2^{3x+1} - 17 \cdot 2^{2x} + 2^{x+3} = 0

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Solution:
We can re-write the equation as 2x(2⋅(2x)2−17⋅2x+8)=02^{x}\left(2 \cdot (2^{x})^{2} - 17 \cdot 2^{x} + 8\right) = 0, or 2⋅(2x)2−17⋅2x+8=02 \cdot (2^{x})^{2} - 17 \cdot 2^{x} + 8 = 0. Make the substitution y=2xy = 2^{x}. Then we have 2y2−17y+8=02y^{2} - 17y + 8 = 0, which has solutions (by the quadratic formula) y=17±289−644=17±154=8,12y = \frac{17 \pm \sqrt{289 - 64}}{4} = \frac{17 \pm 15}{4} = 8, \frac{1}{2}, so 2x=8,122^{x} = 8, \frac{1}{2} and x=3,−1x = 3, -1. The product of these numbers is −3-3.

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