OG详解-OG6 数学2 Q1

Choice B is correct. Substituting 72 for f(x) in the given function yields 72 = 8x. Dividing each side of this equation by 8 yields 9 = x. Therefore, f(x) = 72 when the value of x is 9.
Choice A is incorrect. This is the value of x for which f(x) = 64, not f(x) = 72. Choice C is incorrect. This is the value of x for which f(x) = 512, not f(x) = 72. Choice D is incorrect. This is the value of x for which f(x) = 640, not f(x) = 72.