So if f(x) -> inf and g(x) -> 0, then f(x) * g(x) would represent inf * 0. So if f(x) = x, and g(x) = 1/x as x gets large, then f(x) is always 1. If f(x) = x^2 and g(x) = 1/x as x gets large, then f(x) is x, which -> inf. So we'll get different limits, depending on how f and g vary.