Cos2a=1/2. чему равен cosa?

Question

Cos2a=1/2. чему равен cosa?

Answer 1

Cos2a=2cos²-1
cosa=(cos2a+1)/2=+-√3/2

Answer 2

Cos2a =1/2; 2a=arccos1/2+2Πn; a= (arccos1/2+2Πn)/2