Question

Answer 1

\(1\\ A=\left(x-2\right)\left(x-2\right)-\left(x-1\right)\left(x+1\right)\\ =\left(x-2\right)^2-\left(x^2-1\right)=x^2-4x+4-x^2+1\\ =-4x+5\\ Thay.x=21.vào.A.thu.gọn:A=-4.\left(21\right)+5=-84+5=-79\\ Vậy:A=79.khi.x=21\)

Answer 2

\(2\\ B=\left(x-1\right)\left(x-7\right)-\left(2x-6\right)\left(x-1\right)\\ =\left(x-1\right)\left(x-7-2x+6\right)=\left(x-1\right)\left(-x-1\right)=-\left(x-1\right)\left(x+1\right)=-\left(x^2-1\right)\\ Thay.x=0.vào.B.thu.gọn:B=-\left(0^2-1\right)=-\left(-1\right)=1\\ Vậy:B=1.khi.x=0\)

Answer 3

1) A = (x - 2)(x - 2) - (x - 1)(x + 1)

= x² - 4x + 4 - x² + 1

= -4x + 5

= -84 + 5

= -79

2) B = (x - 1)(x - 7) - (2x - 6)(x - 1)

= x² - 8x + 7 - 2x² + 8x - 6

= -x² + 1

= 1