Function defined on the set of real numbers by a relationship of the form

*y*=*kx*where*k*is a non-zero real number.The graph of a direct variation function always passes through the origin (0, 0) of the Cartesian plane.

### Example

This is the graph of the function *f* defined by the rule *f*(*x*) = -2*x*.

The rate of change of this function is −2.

Since the axes of the Cartesian plane are orthogonal, we can say that the slope is –2.