# Video: Differentiation for Inverse Functions

If 𝑓(1/2) = −6, 𝑓′(1/2) = 3, and 𝑎 = −6, find (𝑓⁻¹)′(𝑎).

01:31

### Video Transcript

If 𝑓 of a half equals negative six, 𝑓 prime of a half equals three, and 𝑎 equals negative six, find the inverse of 𝑓 prime of 𝑎.

Let’s start by recalling a useful formula for finding the derivative of an inverse function. That is the inverse of 𝑓 prime of 𝑎 equals one over 𝑓 prime of the inverse of 𝑓 of 𝑎. For our question, 𝑎 equals negative six. So we can substitute this into the formula. So we’re going to need to find what 𝑓 prime of the inverse of 𝑓 of negative six is. Well, we can find what the inverse of 𝑓 of negative six is using the information in the question and the definition of the inverse function.

The definition of the inverse function says that 𝑓 of the inverse of 𝑓 of 𝑎 equals 𝑎. And so 𝑓 of the inverse of 𝑓 of negative six equals negative six. From the information given in the question, 𝑓 of one-half equals negative six. So the inverse of 𝑓 of negative six must be equal to a half. If we substitute this back into our workings, we find that the inverse of 𝑓 prime of negative six equals one over 𝑓 prime of one-half. From the question, we’re told that 𝑓 prime of one-half equals three. And so we find that the inverse of 𝑓 prime of negative six equals one over three.