Question

How do you use the angle sum or difference identity to find the exact value of `cos ((5pi)/ 12)`?

Answer 1

cos (a - b) = cos a.cos b + sin a.sin b
cos (5Pi/12) = cos (Pi/2 - Pi/12) = cos Pi/2.cos Pi/12 + sin Pi/2.sin Pi/12
= (0).(0.97) + (1).(0.26) = 0.26.
cos (5Pi/12) = 0.26

Answer 2

You need to split the `(5pi)/12` into two angles whose trig value you know.

`(1 pi)/12 +(4 pi)/12`;will work if you know the trig values for `pi/12`

Or try

`(2pi)/12 + (3pi)/12`

You should know the trig values for `pi/6` and `pi/4`.

Now use the sum identity for cos.