Direct Variation / Directly Proportional

y is directly proportional to x, yx:

$y = kx$

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

Joint Variation / Jointly Proportional

y is directly proportional to x and z:
$y = kxz$

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

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

k = constant of proportionality

Variation to nth power of x and mth power of z

y is directly proportional to the square of x and varies inversely to the cube of z:

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

k = constant of proportionality

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