Question

How do you use the angle sum or difference identity to find the exact value of `cos(45-60)`?

Answer 1

`cos(45^o-60^o)=cos(-15^o)=(1+sqrt3)/(2sqrt2`

Explanation:

The related identity is `cos(A-B)=cosAcosB+sinAsinB`

As such `cos(45^o-60^o)=cos45^ocos60^o + sin45^osin60^o`

= `1/sqrt2xx1/2+1/sqrt2 xx sqrt3/2`

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