Sometimes a graph in the plane is defined by an equation of the form , but this equation cannot be put into the form . In such a case we cannot compute a derivative by the ordinary methods. Nevertheless, on some well-chosen intervals, it is possible to do it by implicit differentiation. We will see that on examples.

We will not do it here in this way. Consider the equation of :

We differentiate each side with respect to :

If , we have:

E.g. the slope of the tangent to at the point whose coordinates are is equal to .

At every point such that , is a differentiable fuction of and we have:

E.g., the slope of the tangent to at the point whose coordinates are is equal to 0.