Bài `1` :
Ta có :
`(x+y)^2 = 12^2 =144`
Mà `(x+y)^2 = x^2 +2xy +y^2`
`-> x^2 + 2xy +y^2 =144`
`-> x^2 + 2 . 35 + y^2= 144`
`-> x^2 + 70 + y^2 =144`
`-> x^2 +y^2 = 74`
$\\$
Bài `2` :
`89^2+ 11^2 + 22^2 . 89`
`= 89 . 89 + 11 . 11 + 11^2 . 2^2 . 89`
`= (89 . 89 + 2 . 11 . 89 +11^2) - 22 . 89 + 11^2 . 2^2 . 89`
`= (89+11)^2 + 22 . 89(22 -1)`
`= (89+11)^2 +22 . 89 . 21`
`= 100^2 + 41118`
`= 51118`
$\\$
Bài `3` :
`\text{a)}`
`16x^2 - 72x +36`
`= 16^2 - 48x + 36 - 24x`
`= (4x)^2 - 2 . 4x . 6 + 36 - 24`
`= (4x-6)^2 -24`
$\\$
`\text{b)}`
`-2x +x^2+1`
`= x^2 - 2x +1`
`= x^2 - 2 . x . 1 + 1`
`= (x-1)^2`
