Question

How do you express `cos( (5 pi)/4 ) * cos (( pi) /6 )` without using products of trigonometric functions?

Answer 1

`-sqrt6/4`

Explanation:

Trig table, and unit circle -->
`cos (pi/6) = sqrt3/2`
`cos ((5pi)/4) = cos (pi/4 + pi) = - cos (pi/4) = -sqrt2/2`.
Product `P = (-sqrt2/2)(sqrt3/2) = -sqrt6/4`