text: According to integration by parts\nint(f(x)g(x)) = f(x)int(g(x))-int(f'(x)int(g(x))\nNow, consider \nint (f(x)g''(x) )dx\n=f(x)g'(x)-int(f'(x)g'(x)) since int(f'(x)) = f(x)\n=f(x)g'(x)- { f'(x)g(x) - int(f''(x)g(x)) } limit from 0 to a\n={f(x)g'(x) - f'(x)g(x) + int(f''(x)g(x)) } limit from 0 to a\n={f(a)g'(a) - f'(a)g(a)} - {f(0)g'(0) - f'(0)g(0)} + {int(f''(x)g(x)) } limit from 0 to a\n={f(a)g'(a) - f'(a)g(a)} + {int(f''(x)g(x)) } limit from 0 to a\nSince f(0) = g(0) = 0\n\nThats the proof.