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Joint Variation
Jointly Proportional

When we say z is jointly proportional to a set of variables, it means that z is directly proportional to each variable taken one at a time.

If z varies jointly with respect to x and y, the equation will be of the form z = kxy (where k is a constant).

Equation: c = 5ab

Variable c is jointly proportional to a and b. That means c is directly proportional to both a and b.

Doubling a causes c to double. Doubling b causes c to double. Doubling both a and b causes c to quadruple.

 a b c 1 1 5 2 1 10 1 2 10 2 2 20