a) `x^2 + 2.x`
`= x . x + 2 . x`
`= x(x + 2)`
b) `3x^2 - xy + 9x - 3y`
`= ( 3x^2 + 9x ) - ( xy + 3y )`
`= ( 3.x.x + 3.3.x ) - y( x + 3 )`
`= 3x ( x + 3 ) - y ( x + 3 )`
`= ( 3x - y ) ( x + 3 )`
Tính nhanh :
`123^2 - 246 . 23 + 529`
`= 123 . 123 - 123 . 2 . 23 + 529`
`= 123(123 - 2.23) + 529`
`= 123 . 77 + 529`
`= 10000`