Question

How do you use the angle difference identity to find the exact value of `cos-15`?

Answer 1

`cos(-15^o)=0.9659`

Explanation:

`cos(-15^o)=cos(30^o-45^o)`

= `cos30^ocos45^o+sin30^osin45^o`

= `sqrt3/2xx1/sqrt2+1/2xx1/sqrt2`

= `(sqrt3+1)/(2sqrt2)`

= `0.9659`