CMR các số sau là số chính phương a) A= 111...1-222...2 ( 2n chữ số 1 và n chữ số 2)

`A=\underbrace{111...1}_{2n}-\overbrace{222...2}^{n}`

`=1\underbrace{000...0}_{n-1}1.\underbrace{111...1}_{n}-2.\overbrace{111...1}^{n}`

`=(1\underbrace{000...0}_{n-1}1-2).\underbrace{111...1}_{n}`

`=\underbrace{999...9}_{n}.\underbrace{111...1}_{n}`

`=\underbrace{333...3}_{n}.3.\underbrace{111...1}_{n}`

`=\underbrace{333...3}_{n}.\underbrace{333...3}_{n}`

`=(\underbrace{333...3}_{n})^2`

Vì `\underbrace{333...3}_{n}∈\mathbbZ`

`⇒A` là `SCP` `(đpcm)`