已知数列{an}满足a1=1,an+1(n+1为下标)=an(n为下标)+n+1求a6
问题描述：
已知数列{an}满足a1=1,an+1(n+1为下标)=an(n为下标)+n+1求a6
答
a(n+1)=an+n+1
a(n+1) -an= n+1
an - a(n-1) = n
an - a1 = 2+3+..+n
an = 1+2+..+n = n(n+1)/2
a6 = 21