13题: 一开始看的一头雾水,后在网上搜到差不多的,大家看了可能就明白了,我估计是狗主记忆稍微有点偏差: 搜到内容如下: The sum of the first n odd numbers is always n square For example, 1=1 1 + 3 = 4 1 + 3 + 5 = 9 1 + 3 + 5 + 7 = 16 sum_odd(n) = n(2a + (n-1)d)/2 = n(2 + (n-1)2)/2= n(1 + n - 1) = n^2 So, the sum of the first n odd numbers is n^2. 网址如下: http://blog.jgc.org/2008/02/sum-of-first-n-odd-numbers-is-always.html