Let P = {x0,x1,x2, ... ,xn} be a regular partition of the interval [a,b]. (Show that if f is continuous and decreasing on [a,b], then... Uf(P) - Lf(P) = [f(a) - f(b)]▲x
Notice that this is a regular partition. So .
Also this is a decreasing function which means .
Can you see how to finish?