Suppose the tangent to circle C2 at point P meets the line AB at point T1 and the tangent to circle C1 at point P meets AB at point T2 (see Fig.). Therefore, PT2 is at right angles at AP and PT1 is at right angles to PB. It is given that ΔT1T2 = θ. Now,
Therefore, using cosine rule to ΔPAB, we get