Найдите cos a,если cos a/2=-1/√5,π

Question

Найдите cos a,если cos a/2=-1/√5,π

Answer 1

cosa = cos^2(a/2) - sin^2(a/2)

sin^2(a/2) = 1 - cos^2(a/2) = 1 - 1/5 = 4/5

cosa = 1/5 - 4/5 = -3/5