1+2+1=4, ->2^2=4
1+2+3+2+1=9, ->3^2=9
1+2+3+4+3+2+1=16; ->4^2=16
1+2+3+...+n-1+n+n-1+...+2+1 , -> n^2
1+2+3+4+...+100+99+...+2+1=100^2=10000