原式=(x+y)/(√x-√y)-(2xy)/(x√y-y√x)
=(x+y)/(√x-√y)-(2xy)/[√(xy)(√x-√y)]
=(x+y)/(√x-√y)-2√(xy)/(√x-√y)]
=[(x+y)-2√(xy)]/(√x-√y)
=[x-2√(xy)+y]/(√x-√y)
=(√x-√y)²/(√x-√y)
=√x-√y
当x=8,y=18时
原式=√8-√18
=2√2-3√2
= -√2