Mathematics-Joint Variation

Joint Variation or Direct variation is defined as a quantity that varies directly with product of 2 or more other quantities.
Such a proportionality is known as direct proportionality.

Direct Variation means changes in one quantity affects the other quantity also. Increase or decrese  in one quantity affects increase or decrese in the other quantity too.

Eg ) Z = 5xy
In the above case Z increase or decrease when any one of the x & y increase or decrease. So we can say that Z is Jointly proportional to both x & y