已知有理数x,y,z满足：x-z-2的绝对值+3x-6y-7的平方+3y+3z-4的平方=0,
x-z-2=0 x-z=2
3x-6y-7=0 x-2y=7/3
3y+3z-4=0 y+z=4/3
x=3
y=1/3
z=1
x的3n次方y的3n+1次方-z
=3^3n*1/3^3n+1-1
=(3*1/3)^3n*1/3-1
=1/3-1
=-2/3