x/x-y*y2/x+y-x4y/x4-y4/x2/x2+y2

原式=(xy²)/(x+y)(x-y)-[x^4y/(x²+y²)(x²-y²)][(x²+y²)/x²]
=(xy²)/(x+y)(x-y)-x²y/(x²-y²)
=-xy(x-y)/(x+y)(x-y)
=-xy/(x+y)

x>0,y>0
20=5x+7y>=2√(5x*7y)
所以√(35xy)<=20/2=10
平方
35xy<=100
xy<=20/7
所以最大值=20/7