1) Direct variation -- as the independent variable increases so does the dependent variable.

The amount of money a person who is paid hourly makes varies directly with the number of hours worked.

P=kh where P is the pay, h the number of hours, and k, the proportionality constant, is the hourly wage.

2) Inverse variation -- as the independent variable increases, the dependent variable decreases or vice versa.

The worth of a car varies inversely with its age.

`W=k/a` where W is the worth of a car at age a and k is the proportionality constant.

3) Joint variation -- the dependent variable varies directly with two or more independent variables.

The gravitational pull between two objects varies jointly with their masses and inversely with the square of the distance between them.

`F=g(m_1m_2)/d^2` where g is the gravitational constant.