(m+2)^2-m^2=(m+1)4,展开一下就可以验证了
(n+1)^2-(n-1)^2=4n将(n+1)^2展开：(n+1)^2=n^2+2n+1将(n-1)^2展开：(n-1)^2=n^2-2n+1两式作差：左边=(n+1)^2-(n-1)^2=n^2+2n+1-(n^2-2n+1)=4n
