n(n+3)和(n+1)(n+2)两个都是偶数，先计算出来各自除2，最后乘起来再除2
n(n+3) = n^2+3n
(n+1)(n+2)=n^2+3n+1

所以令a=n(n+3)/2
则最后结果是a(a+1)/2