E-STAR - Student

The quantity y is said to be in direct proportion to the quantity x, written as y α x, if there exists a nonzero constant number k such that y = kx, where k is called the constant of variation or constant of proportionality.
The quantity y is said to be inverse proportion to the quantity x, written as y

if there exists a constant number k such that

The number k is called the constant of variation
If more than two variables are related directly or one variable changes with the change in the product of two or more variables it is called as joint variation. If x is in joint variation with y and z, written as x α yz, if there exist a constant k such that x = kyz.
A quantity y is partly constant and partly varies directly as x, then the law of variation can be expressed as y α (a + x). That is, y = a + kx, where a and k are constants.