Question

计算：（x-3y)(x+7y)化简：(-4x+1)(2x+3)-(-8x)(x+1)先化简,再求值：2（x-8)(x+5

Answer 1

1.（x-3y）（x+7y）=x^2+7xy-3xy-21y^2=x^2-21y^2+4xy

2.(-4x+1)(2x+3)-(-8x)(x+1)=-8x^2-12x+2x+3+8x^2+8x=-2x+3

3.2(x-8)(x+5)-(2x-1)(x+2)=2(x^2+5x-8x-40)-(2x^2+4x-x-2)=2x^2-6x-80-2x^2-3x+2=-9x-78,当x=-7时,原式=-9*(-7)-78=-15