We start by letting  for which we want to find the derivative .

We have already derived the relationship
 
 

Now, we note that  and so we may continue the above line as
 
 

We evaluate , where y = by looking at the right triangle in which the relationship
 
 

holds.  So, in this triangle the side opposite the angle  equals , the hypotenuse equals , and the side adjacent to the angle equals .

This is a triangle as below
 
 
 

Therefore, we have that  the derivative of is given by
 
 

or,
 

99.