a(n+1)/an=n+2/n改成a(n+1)/an=（n+2）/n这是a(n+1)/an=f(n)的形式，用累乘法：a2/a1=3/1a3/a2=4/2...a(n-1)/a(n-2)=n/(n-2)an/a(n-1)=(n+1)/(n-1)等式左边相乘=等式右边相乘得an/a1=n(n+1)/2an=n(n+1)/2
