4. let f be defined and bounded on a compact rectangle q = la, b] x lc, d| in r. assume that for each fixedy elc, dj, f(r, y) is an increasing function of a and

4. Let f be defined and bounded on a compact rectangle Q = la, b] x lc, d| in R. Assume that for each fixedy elc, dj, f(r, y) is an increasing function of a and for each fixed r e la, b], S(r, y) is an increasing function of y. Prove that f is Riemann integrable on Q.