Introduction to Differentiation - Tangents on a Circle

A Brookes
The values m and m1 show the gradient of the tangent at the point on the circle. Slide points A and B around to see how the gradient changes around the circle. Turn trace on for points A' and B' to see the gradient function appear.
What values are the points A' and B' using to plot the gradient function? What type of equation would you expect the gradient function to have?