(a减b)的2n次方(b减a)(a减b)的(m减1)次方
=(a-b)的2n次方[-(a-b)](a-b)的(m-1)次方
=(a-b)的2n次方[-(a-b)的(1+m-1)次方]
=-(a-b)的(2n+m)次方