∫(0→a){f(x)/[f(x)+f(a-x)]}dx＝∫(a→0){f(a-t)/[f(a-t)+f(t)]}d(a-t)＝∫(0→a){f(a-t)/[f(t)+f(a-t)]}dt＝∫(0→a){f(a-x)/[f(x)+f(a-x)]}dx＝(令) A则2A＝∫(0→a){f(x)/[f(x)+f(a-x)]}dx + ∫(0→a){f(a-x)/[...