证明：
令x=1-t,则dx=-dt,当x=1,t=0,当x=0,t=1
∫(0,1)x^m(1-x)^ndx
=-∫(1,0)(1-t)^m(t^n)dt
=∫(0,1)(1-t)^m(t^n)dt
=∫(0,1)(1-x)^m(x^n)dx
命题成立