Find an exact value cos (19pi/12)

19 π / 12 = 19 * 180° / 12 = 285 °

cos (19 π / 12) = cos 285° = cos ( - 75°) = cos 75°

cos 75° = cos (45° + 30°) = cos 45° cos 30° - sin 45° sin 30° =

= (√2 / 2 · √3/2) - (√2 / 2 · 1/2) = √6/4 - √2/4 =

= (√6 - √2) / 4