令b=a-1
aΔb=a(a+2)-(a+1)-b
```=a^2+2a-a-1-(a-1)
```=a^2+2a-2a-1+1
```=a^2
下面的式子满足上面的推导
(2Δ1)+…+(11Δ10)=2^2+3^2+.11^2=505