x∝yz⇒x=k1⋅yz(k1≠0= variation constant )
⇒y=xk1z.......(1)
Again, y∝zx⇒y=k2⋅zx(k2≠0= variation constant )
⇒xk1z=k2⋅zx[ from (1)] ⇒z2=k1k2⇒z=√k1k2.......(2)
Since k1,k2 are non-zero variation constants,
∴√k1k2 is a non-zero variation constant.
⇒z is a ron-zero variation constant.