原式=(√x-√y)/(√x+√y)
=(√x-√y)^2/(x-y)
=(x+y-2√xy)/(x-y)
x+y=12
x²+2xy+y²=144
x²-2xy+y²-4xy=144
(x-y)²-4xy=144
(x-y)²=180
x>y
x-y=√180
原式=(12-2×3)/√180=6/(6√5)=√5/5