# Use the equation twice to find cos(15) if it known that cos(60)=0.5

### 1 Answer | Add Yours

You need to use the formula of half angle such that:

`cos (alpha/2) = sqrt ((1 +cos alpha)/2)`

You have recommended to use the cosine of the angle of `60^o` .

`cos 30^o = cos (60^o/2) = sqrt ((1 + cos 60^o)/2)`

`cos 30^o = cos (60^o/2) = sqrt ((1 + 0.5)/2)`

`cos 30^o = cos (60^o/2) = sqrt ((1.5)/2)`

`cos 15^o = cos (30^o/2) = sqrt ((1 + sqrt ((1.5)/2))/2)`

`cos 15^o~~` 0.965

**The value of cos `15^o` `~~` 0.965.**