5-25. Given the bivariate cumulative distribution function F(t, s), the joint probability P(a < X ≤

b, Y ≤

d) = F

(b,

d) − F

(a, d). Using this expression, find P 13

< X ≤ 1,Y ≤ 2 3  when f (x, y) = -4xy, 0< x < 1 and 0 < y < 1;

0 elsewhere.