( x - 3)( x +3) - ( 2x - 3)2
= x2 - 32 - [ ( 2x)2 - 2.2x.3 + 9]
= x2 - 9 - 4x2 + 12x - 9
= - 3x2 + 12x - 18
Ta có:\(\left(x-3\right)\left(x+3\right)-\left(2x-3\right)^2\)
\(=\left(x^2-9\right)-\left(4x^2-12x+9\right)\)
\(=x^2-9-4x^2+12x-9\)
\(=\left(x^2-4x^2\right)+12x-\left(9+9\right)\)
\(=\left(-3x^2\right)+12x-18\)