nKrynka Решение
cosa/2 = 7/8
cosa = 2cos²(a/2) - 1
cosa = 2*(7/8)² - 1 = 49/32 - 1 = (49 - 32)/32 = 17/32

nikitokkk Sin a/2=(1-49/64)^0.5=15^0.5/8
cos a=(cos a/2)^2-(sin a/2)^2
cos a=49/64-15/64=34/64=17/32