câu 49: Rút gọn các biểu thức sau 1) x ( x – 2 ) – 5 ( x – 1 ) ( x + 9 ) 2) ( 4x + 3 ) ( 2 – y ) + ( y + 1 ) ( x – 5 ) 3) ( x + 5 ) ( x -1 ) + ( 2

`x(x-2)-5(x-1)(x+9)`

`= x^2 – 2x – 5(x-1)(x+9)`

`= x^2 – 2x – 5x^2 – 40x + 45`

`= -4x^2- 42x+45`

`(4x+3)(2-y)+(y+1)(x-5)`

`= 8x – 4xy + 6 – 3y + (y+1)(x-5)`

`= 8x – 4xy + 6 -3y+xy-5y+x-5`

`= 9x – 3xy + 1 – 8y`

`(x+5)(x-1) + (2x-3)(x-7) – 3(x-1)(x+1)`

`= x^2 + 4x- 5+(2x-3)(x-7)-3(x-1)(x+1)`

`= x^2 + 4x – 5 + 2x^2 – 17x + 21-3(x-1)(x+1)`

`= x^2 + 4x – 5 + 2x^2 – 17x + 21 – 3x^2 + 3`

`= -13x + 19`