Given the following from the image below:

* The green circle has a radius equal to **B**
* The yellow line is tangent to the green circle
* The vertical purple line is parallel to the green line and perpendicular to the yellow line.
* The yellow line is perpendicular to both the green line and the vertical purple line
* The purple point is centered on the green circle's edge
* **A** and **B** are known values

I realize several of these constraints overlap, just trying to be thorough.
Pythagorean's theorem can provide the value of **C**, just to illustrate what I know we can determine already.

What is the formula/equation to determine **D**, where **D** is the perpendicular distance from the tangent yellow line to the arc/circle (at the purple point)?

[![Perpendicular Distance To Point From Tangent](https://i.stack.imgur.com/CLZkg.png)](https://i.stack.imgur.com/CLZkg.png)

**Update**

Replacing previous attempts to illustrate solution now with one that I can now visualize as the correct representation of the answer and comments provided by John

[![Solution Imagery](https://i.stack.imgur.com/tNqr3.png)](https://i.stack.imgur.com/tNqr3.png)