令a=99.9,(n个9)
a+1=10^n,
10a+1=99.91,(n个9)
999.9*99.9+199.9(第一个数有n+1个9,第二,三个数有n个9)
=(10a+9)a+2a+1
=10a²+11a+1
=(10a+1)(a+1)
=99...9100...0
(前面n个9,中间1个1,后面n个0)