1 + 1=2
2 + 2=4
4 + 3=7
7 + 4=11
11+ 5=16
16+ 6=22
22+ 7=29
a第(n-1)项+(n-1)=a第n项
an-a1 = 1+2+3+4+...+(n-1)=1/2*n*(n-1)
an = 1/2*n*(n-1) +1
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