Question

How do you express `cos( (5 pi)/6 ) * cos (( 7 pi) /4 )` without using products of trigonometric functions?

Answer 1

`( -sqrt6/4)`

Explanation:

Trig table of special arcs -->
`cos ((5pi)/6) = - sqrt3/2`
`cos ((7pi)/4) = cos (- pi/4 + 2pi) = cos (-pi/4) = cos (pi/4) = sqrt2/2`

`cos ((5pi)/6).cos ((7pi)/4) = - (sqrt3/2)(sqrt2/2) = -sqrt6/4`