Use an Addition or Subtraction Formula to find the exact value of the expression, as demonstrated in Example 1.
cos
(5π/
12)
cos ( 5 pi /12) =
It is easiest to convert the radians to degrees....so
5pi/ 12 * 180 /pi =
(5/12) (180) = 75°
So
cos (75°) =
cos (30 + 45) =
cos30*cos45 - sin 30 *sin 45 =
( √3/2 ) ( √2/ 2) - (1/2) ( √2/2) =
√6/4 - √2/4 =
√6 - √2
_______
4
+318 
