thu gọn đa thức a. 3x(x-5)-(x-2)7x b. (x+4)(x-4)-2x(3-x) c. (3x-7)(-2x+1)-8x(6-x)

`a) 3x(x – 5) – (x – 2)7x`

`= 3x. x – 3x. 5 – x. 7x + 2. 7x`

`= 3x^2 – 15x – 7x^2 + 14x`

`= (3x^2 – 7x^2) + (-15x + 14x)`

`= -4x^2 – x`

`b) (x + 4)(x – 4) – 2x(3 – x)`

`= x^2 – 4^2 – 2x. 3 + 2x. x`

`= x^2 – 16 – 6x + 2x^2`

`= (x^2 + 2x^2) – 6x – 16`

`= 3x^2 – 6x – 16`

`c) (3x – 7)(-2x + 1) – 8x(6 – x)`

`= (3x – 7). -2x + (3x – 7) – 8x. 6 + 8x. x`

`= 3x. -2x + 7. 2x + 3x – 7 – 48x + 8x^2`

`= -6x^2 + 14x + 3x – 7 – 48x + 8x^2`

`= (-6x^2 + 8x^2) + (14x + 3x – 48x) – 7`

`= 2x^2 – 31x – 7`