Rút gọn biểu thức: (x + y + z)3 + (x - y - z)3 + (y - x - z)3 + (z - y - x)3

1) Rút gọn bt:

(x+y+z)3+(x-y-z)3+(y-x-z)3+(z-y-x)3

2)Tìm x,y,z t/m: 9x2+y2+2z2-18x+4z-6y+20=0

3)Cho x/a+y/b+z/c=1 và a/x+b/y+c/z=0 . CMR:

x^2/a^2+y^2/b^2+z^2/c^2=1