y=√（x-27）+√（27-x）+12
x-27>=0
27-x>=0
所以
x=27
y=12
(x+y/√x-√y)-2xy/x√y-y√x
=(x+y/√x-√y)-2√x√y/(√x-√y)
=(√x-√y)²/(√x-√y)
=(√x-√y)
=(√27-√12)
=√3