Question

How do you express `cos( (5 pi)/6 ) * cos (( 11 pi) /6 )` without using products of trigonometric functions?

Answer 1

`-3/4`

Explanation:

Trig table --> `cos ((5pi)/6) =cos(pi-pi/6)=-cospi/6=-sqrt3/2`
`cos ((11pi)/6) = cos (-pi/6 + (12pi)/6) = cos (-pi/6 + 2pi) =`
`cos (-pi/6) = cos (pi/6) = sqrt3/2`
The product can be expressed as:
`P = (-sqrt3/2)(sqrt3/2) = - 3/4`