99.9×99...9+199...9
=(99…9)²+2*99…9+1
=(99…9+1)²
=10^(2n) 99....9×99...9+199...9=(99…9)²+2*99…9+1为什么是2*99…9+1199...9=2*99…9+1为了凑完全平方式