(n-1)^2 + (m-2)(m-2n)=(n-1)(n-1) + [m(m-2n) - 2(m-2n)]=(n^2 - 2n +1) +[(m^2 - 2mn)-2m + 4n]=n^2 -2n + 1 + m^2 -2mn - 2m + 4n=n^2 - 2n +1 + 4n + m^2 -2m - 2mn即可得到