**Direct Variation / Directly Proportional**

$y = kx$

k = constant of proportionality

y varies directly as x is another statement equivalent to the above statement.

**Inverse Variation / Directly Proportional**

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

k = constant of proportionality

y varies inversely with x holds the same meaning as the sentence above.

**Joint Variation / Jointly Proportional**

$y = kxz$

y is directly proportional to x and inversely proportional to z:

$y = \dfrac{kx}{z}$

k = constant of proportionality

**Variation to n ^{th} power of x and m^{th} power of z**

$y = \dfrac{kx^2}{z^3}$

k = constant of proportionality