If cos B =1/7 then what is the positive value of cos 1/2 B, in simplest radical form with a rational denominator?

Question

Help me, please! It's due at 1:30!!!

Answer 1

cos(B/2) = sqr((cosB+1)/2)

cos(B/2) = sqr((1/7+1)/2)= sqr(8/7/2) = sqr(4/7) = 2sqr(1/7 = (2/7)sqr7 = 0.756

cos(B/2) = 0.756

alternative method

cosB = 1/7 so B = 81.8 degrees

B/2 = 40.9 degrees

cos(B/2) = cos(40.9) = 0.756