Question

How do you express `cos( (15 pi)/ 8 ) * cos (( 2 pi) /3 )` without using products of trigonometric functions?

Answer 1

`-(1/2)cos (pi/8)`

Explanation:

`P = cos ((15pi)/8)cos ((2pi)/3)`
trig table gives -->cos (2pi)/3 = - 1/2
`cos ((15pi)/8) = cos (-pi/8 + (16pi)/8) = cos (-pi/8 + 2pi) =`
`= cos (-pi/8) = cos (pi/8)`
Therefor, P can be expressed as:
`P = -(1/2)cos (pi/8)`