1+2+1=
1+2+3+2+1=
1+2+3+4+3+2+1=
1+2+3+4+5+4+3+2+1=
……
16
25
1+2+3+…+99+100+99+…+3+2+1=
10000
1+2+3+…+(n-1)+n+(n-1)+…+3+2+1 = n2
做一做
= 22
= 32
= 42
= 52
