So the square of (x+y+z) =x square +y square +z square +2xy+2yz+2xz= 1.

At this time, 2 (the square of x+the square of y+the square of z) =2-4xy-4yz-4xz is greater than or equal to 2xy+2yz+2xz.

So xy+yz+xz is less than or equal to 1/3.

So the square of x+the square of y+the square of z = 1-2xy-2yz-2xz is greater than or equal to 1/3.