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