Find the exact value of the trig function below.
cos a/2 if cosa = - 31/49 (assume cos a/2 is negative)
we know cos (2x) = 2cos^2 x - 1
so cos a = 2cos^2 (a/2) - 1
-31/49 + 1 = 2cos^2 (a/2)
18/49 = 2cos^2 (a/2)
9/49 = cos^2 (a/2)
cos (a/2) = ± 3/7
but we were to assume that cos (a/2) is negative, so
cos (a/2) = -3/7
Check with your calculator