SAT Math · Nonlinear equations in one variable · medium

What value of x satisfies 5^x+3 = 25^x-2?

What value of xx satisfies 5x+3=25x−25^{x+3} = 25^{x-2}?

Answer and explanation

Answer: D: 7

Rewrite both sides with base 5. Since 25=5225 = 5^2, the right side is (52)x−2=52(x−2)(5^2)^{x-2} = 5^{2(x-2)}. Equate exponents: x+3=2(x−2)=2x−4x + 3 = 2(x - 2) = 2x - 4. Subtract xx and add 4: 7=x7 = x, choice D. Check: 5105^{10} on the left and 255=51025^5 = 5^{10} on the right. Choice A (1) gives 545^4 versus 25−125^{-1}. Choice B (−1-1) gives 525^2 versus 25−325^{-3}. Choice C (5) gives 585^8 versus 253=5625^3 = 5^6. A common error is writing the right exponent as 2x−22x - 2, forgetting to distribute the 2 over both terms.

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