Can i asking what is the fomula of cos^2 1/2A ？

Ineed i been stuck on this question since yesterday

*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

Ab and bc are tangent to circle d. find x is ab = 3x + 8 and bc = 26. find x