Tolong bantu ya guys, diketahui sin alfa 12per13 tentukan cos 1per2 alfa?

Sin x = 12/13
cos (x/2) = …?

cos x = (√13²-12²)/13
         = 5/13

         cos x = cos² (x/2) – sin² (x/2)
          5/13 = cos² (x/2) – (1 – cos² (x/2))
          5/13 = 2cos² (x/2) – 1
2cos² (x/2) = 1 + 5/13
  cos² (x/2) = 9/13
   cos (x/2) = (3.√13)/13 atau -(3.√13)/13