若有理数x,y,zx,y,zx,y,z满足x+y−1+z−2=x+y+z2\sqrt{x}+\sqrt{y-1}+\sqrt{z-2}=\dfrac{x+y+z}{2}x+y−1+z−2=2x+y+z，求(x−yz)3(x-yz)^{3}(x−yz)3
