ACT Math · Polynomial and rational functions

What are the vertical asymptotes of f(x) = (x+2)/(x²-16)?

What are the vertical asymptotes of f(x)=x+2x2−16f(x) = \dfrac{x+2}{x^2-16}?

Answer and explanation

Answer: A: x = 4 and x = -4

A vertical asymptote occurs where the denominator is zero and the factor does not cancel with the numerator. Set x2−16=0x^2-16=0 to get (x−4)(x+4)=0(x-4)(x+4)=0, so x=4x=4 or x=−4x=-4. The numerator, x+2x+2, is not zero at either value, so nothing cancels and both are asymptotes, choice A. Choice B (x=−2x=-2) is where the numerator is zero, which gives an xx-intercept, not an asymptote. Choice C (x=16x=16) confuses x2=16x^2=16 with x=16x=16. Choice D (x=4x=4 only) finds just one of the two roots of x2=16x^2=16.

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