Based on the diagram, the following relation holds.
Keep in mind that this theorem is true also in the case that is a secant.
Consider a diameter Since is tangent to the circle at then
In addition, since is a diameter, is a semicircle and then its measure is The Arc Addition Postulate allows setting the following equation. Since is a central angle, its measure is half the measure of the intercepted arc. That is, or equivalently, Similarly, the Angle Addition Postulate can be used to establish the following equation. Next, multiply the equation above by and add it to the previous equation. Solving the last equation by gives the first desired equation.
Once more, the Arc Addition Postulate and the Angle Addition Postulate can be applied to set the following pair of equations. Recall that and Next, multiply Equation by and add it to Equation Finally, by solving the resulting equation for the second desired equation is obtained.