- 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:
(because the coordinates of Q also satisfy the relationship)So the gradient of PQ is
The limit of this expression as is then clearly .