Calculating the radius of a cylindrical milling cutter

Knowing the geometry parameters (cutting depth b and milling cut boundary a) we can draft an isosceles triangle. This triangle has two sides x equal to the required cutter's radius .

At the isosceles triangle we draft an intersecting line through the third side,  creating two equal right triangles. Then we draft a vertical line from the corner between the large side  x (equal the radius) and the small third one c to the other large side, creating this way another right triangle. The new right triangle is  similar with the other ones (one side common, two sides vertical).

As all the triangles are similar then: