(am+bn)/(m+n)
(x-1)^2/4x *(2x2)/(x-x^2) = (x-1)^2/4x * 4/x(1-x) = (1-x)^2/4x * 4/x(1-x)=(1-x)/x^2
(16-m^2)/(16+8m+m^2)÷(m-4)/(2m+8)*(m^2-4)/(m+2)
=(m+4)(m-4)/(m+4)^2 ÷(m-4)/2(m+4)*(m+2)(m-2)/(m+2)
=(m-4)/(m+4) * 2(m+4)*/(m-4))* (m-2)
=2(m-2)=2m-4
[(x^2-5x+6)/(12-7x+x^2)]^2÷[(x^2+3x+2)/(x^2-2x-8)]^2
=[(x-2)(x-3)/(x-3)(x-4)]^2 ÷ [(x+1)(x+2)/(x+2)(x-4)]^2
=[(x-2)/(x-4)]^2 ÷ [(x+1)/(x-4)]^2
=(x-2)^2/(x-4)^2 * (x-4)^2/(x+1)^2
=(x-2)^2/(x+1)^2
x=-5 原式= (-5-2)^2/(-5+1)^2 =(-7)^2/(-4)^2 = 49/16