m为偶数时候 x^m-y^m=(x^(m/2)+y^(m/2))(x^(m/2)-y^(m/2)) m为奇数时候 x^m-y^m=(x-y)(x^(m-1)+x^(m-2)*y+x^(m-3)*y^2)+.+x*y^(m-2)+y^(M-1)