joint variation

Direct Variation / Directly Proportional

y is directly proportional to x, y ∝ x:

$y = kx$

k = constant of proportionality
y varies directly as x is another statement equivalent to the above statement.

Inverse Variation / Directly Proportional

y is inversely proportional to x, y ∝ 1/x:

$y = \dfrac{k}{x}$

k = constant of proportionality