X^4-X^2-72
=(x^2+8)(x^2-9)
=(x^2+8)(x+3)(x-3)
(X+2)(X+3)+3X+1
=x^2+8x+7
=(x+1)(x+7)
ac-bc+a^2-ab=(a-b)c+a(a-b)=(a-b)(a+c)
已知a-b=3,b+c=-5,两式相加有a+c=-2
所以原式=3*(-2)=-6
X^4-X^2-72
=(x^2+8)(x^2-9)
=(x^2+8)(x+3)(x-3)
(X+2)(X+3)+3X+1
=x^2+8x+7
=(x+1)(x+7)
ac-bc+a^2-ab=(a-b)c+a(a-b)=(a-b)(a+c)
已知a-b=3,b+c=-5,两式相加有a+c=-2
所以原式=3*(-2)=-6