Question

Use a product-to-sum identity to rewrite the expression cos 40° cos 20° ?

Thanks

Answer 1

`(cos 20 + cos 40)/2`

Explanation:

Use trig identity:
`cos a.cos b = (cos (a - b) + cos (a + b))/2`
In this case:
`cos 40.cos 20 = (1/2)[cos (30 - 10) + cos (30 + 10)]`
`= (cos 20 + cos 40)/2`