If x,y,zx, y, zx,y,z form an arithmetic sequence, which of the following is always true?
x+z=2yx+z = 2yx+z=2y
x−y=z−yx-y = z-yx−y=z−y
x2+z2=2y2x^2+z^2 = 2y^2x2+z2=2y2
xz=y2xz = y^2xz=y2