SAT Math · Nonlinear equations in one variable · medium

What value of x satisfies 8^x+1 = 32^x-2?

What value of xx satisfies 8x+1=32x−28^{x+1} = 32^{x-2}?

Answer and explanation

Answer: A: 13/2

Rewrite both sides with the same base. Since 8=238 = 2^3 and 32=2532 = 2^5, the equation becomes 23(x+1)=25(x−2)2^{3(x+1)} = 2^{5(x-2)}. Equal bases with equal values mean the exponents are equal: 3x+3=5x−103x + 3 = 5x - 10. Subtract 3x3x and add 10: 13=2x13 = 2x, so x=132x = \frac{13}{2}, choice A. Choice B (112\frac{11}{2}) results from a sign slip, using 3x+3=5x−83x + 3 = 5x - 8 or subtracting 10 incorrectly. Choice C (2) is the number inside the parentheses. Choice D (5) comes from the same mistake, ignoring the factor of 3 and 5 on the exponents, or mismatching bases.

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