(1) [(x-y)/a]^4*(-a/(x-y)^5=a/y-x
(2)(2x+1)/(x+1)*(1-x^2)/(1-4x^2)*1/(x-1)=(2x+1)(1-x)(1+x)/(x+1)(1+2x)(1-2x)(x-1)=1/(2x-1)