It has to do with calculus. If the probability of a given result approaches zero as the result itself approaches positive or negative infinity, then the area un

It has to do with calculus. If the probability of a given result approaches zero as the result itself approaches positive or negative infinity, then the area under the curve approaches 1. Imagine 1/2 + 1/4 + 1/8 ... to infinity. The sum approaches 1 as the denominator approaches infinity. With calculus, we can determine that with mathematical methods.