What's the common difference of the sequence below

Given: circle o, cd is diameter, chord ab || cd. arc ab = 74°, find arc ca.

*let m∠cob = 50°30’, m∠aob = 70° and m∠aoc = 20°30’. could point c be in the interior of ∠aob? why? a. point c could be the interior of aob but it is not the only case b. point c is the interior of aob c. point c is not the interior of aob d. the given is not possible for the plane geometry answer

What is the product? a-3/15a × 5/a-3