Thu gọn các biểu thức sau:
$ 1) 2x(x - 5) - (x - 2)^2 - (x + 3)(x - 3) $
$ = 2x^2 - 10x - (x^2 - 4x + 4) - (x^2 - 9) $
$ = 2x^2 - 10x - x^2 + 4x - 4 - x^2 + 9 $
$ = (2x^2 - x^2 - x^2) + ( - 10x + 4x) + (-4 + 9) $
$= -6x + 5 $
$ 2) (x + 1)^2 + 3(x + 5)(x - 5) - (2x - 1)^2 $
$= x^2 + 2x + 1 + 3(x^2 - 25) - (4x^2 - 4x + 1) $
$= x^2 + 2x + 1 + 3x^2 - 75 - 4x^2 + 4x - 1) $
$= (x^2 + 3x^2 - 4x^2) + (2x + 4x) + (1 - 75 + 1) $
$= 6x - 75 $
