Find the exact value of the trigonometric function at the gi

Find the exact value of the trigonometric function at the given real number.

(a) cos(19/6)

(b) cos( 7/6)

(c) cos( 11/6)

Solution

(a)

cos(19/6)

=cos((12+7)/6)

=cos(2+(7/6))

=cos(7/6)

=-cos(/6)

=-(3)/2

(b) cos( 7/6)

=cos(2 (7/6))

=cos(5/6)

=-cos(/6)

=-(3)/2

c) cos( 11/6)

=cos(2 (11/6))

=cos(/6)

=(3)/2