The result follows from the inequality between the arithmetic and geometric mean. Since you must have \(\displaystyle x>0,y>0,z>0\). first multiply both sides by \(\displaystyle xyz\) and divide by \(\displaystyle 3\). Then let \(\displaystyle w_1=(xy)^{3/2},w_2=(yz)^{3/2},w_3=(xz)^{3/2}\). Then note that the left-hand side is simply the arithmetic mean, and the right-hand side is the geometric mean of the \(\displaystyle w_i\).