Question

How do you find the exact functional value sin 110° sin 70° - cos 110° cos 70° using the cosine sum or difference identity?

Answer 1

The value of this expression is `1`

Explanation:

In this task we have an expression like:

`sinxsiny-cosxcosy`.

This can be rewritten as:
`-cosxcosy+sinxsiny=-(cosxcosy-sinxsiny)`.

Expression in brackets is the right side of `cos(x+y)` identity, so our expression can be written as

`-cos(x+y=-cos(110+70)=-cos180=-(-1)=1`