ACT Math · Polynomial and rational functions

For the function f(x) = (x²-16)/(x-4), what is the value that f(x) approaches as x approaches 4, even though f(4) is undefined?

For the function f(x)=x2−16x−4f(x) = \dfrac{x^2-16}{x-4}, what is the value that f(x)f(x) approaches as xx approaches 4, even though f(4)f(4) is undefined?

Answer and explanation

Answer: C: 8

Factor the numerator: x2−16x−4=(x−4)(x+4)x−4=x+4\frac{x^2-16}{x-4}=\frac{(x-4)(x+4)}{x-4}=x+4 for x≠4x\ne4. The function has a hole at x=4x=4, and the value it approaches is 4+4=84+4=8, choice C. Choice A (00) is the numerator's value at x=4x=4 with no regard for the zero in the denominator. Choice B (44) is the xx-value of the hole, not the height. Choice D (1616) is the constant in the numerator. Because the factor x−4x-4 cancels, there is no vertical asymptote here.

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