Hello bobsanchezThe usual method is to let the coordinates of the point P be and Q be a neighbouring point at . Then the gradient of the line PQ is:

You then:

- Use the relationship between and (which you will have been given) to write down an expression for in terms of and

- Eliminate from the expression for the gradient, by using this relationship

- Simplify the result, and hence find the limit of this expression as .

For example, if we are told that the relationship between and is:

Then

(because the coordinates of Q also satisfy the relationship)So the gradient of PQ is

The limit of this expression as is then clearly .

Grandad