y有意义
x-8>=0且8-x>=0
所以x=8
y=18
（x+y）-[（√x+√y）分之2x√（y）+2y√x]
=（x+y）-[（√x+√y）分之2√（xy) (√y+√x)]
=x+y-2√（xy)
=8+18-2√144
=26-24
=2