(x/x+y +2y/x+y)乘以xy/ x+2y除以(1/x+1/y)=(x+2y)/(x+y)*xy/(x+2y)/[(x+y)/(xy)]=xy/(x+y)*xy/(x+y)=(xy)^2/(x+y)^2（3x^2/4y)^2乘以2y/3x+ x^2/2y^2除以2y^2/x=9x^4/16y^2*2y/3x+x^2/2y^2*x/2y^2=3x^3/8y+x^3/4y^4...