先化简,再求值:x^2-4y^2/4x^2-8xy,其中x=1/2,y=1/4; 2a^2-2ab/a^2-2ab+b^2,其中a=2,

问题描述:

先化简,再求值:x^2-4y^2/4x^2-8xy,其中x=1/2,y=1/4; 2a^2-2ab/a^2-2ab+b^2,其中a=2,

x^2-4y^2/4x^2-8xy=(x+2y)(x-2y)/4x(x-2y)=(x+2y)/4x=1/4+y/2x=1/4+1/4=1/2
2a^2-2ab/a^2-2ab+b^2=2a(a-b)/(a-b)^2=2a/(a-b)=4