x的2次方+y的2次方+4分之5=2x+y
x²-2x+1+y²-y+1/4=0
(x-1)²+(y-1/2)²=0
x-1=0 y-1/2=0
∴x=1 y=1/2
∴x+y分之xy
=1*1/2/(1+1/2)
=1/2÷3/2
=1/3