一道数学题？

过程与结果如图所示

第n行第1列

1，2, 4, 7, 11,...

a1=2

n>1

an-a(n-1) =  n-1

an-a1 = 1+2+...+(n-1)

an -1 = (1/2)n(n-1)

an = (1/2)n(n-1) +1

an <2021

(1/2)n(n-1) +1 <2021

n(n-1) < 4040

n^2-n -4040 <0

max n =64

a64 = (1/2)(64)(63) +1 =2017

2021-2017+1=5

ie

2021 第64行第5列