Question

How do you use the product to sum formulas to write `10cos75^circcos15^circ` as a sum or difference?

Answer 1

= 5/2

Explanation:

Use trig identity:
`cos a.cos b = (1/2)(cos (a + b) + cos (a - b))`.
In this case:
`10cos 75.cos 15 = (10/2)(cos 90 + cos 60)`
Since cos 90 = 0, and `cos 60 = 1/2`, therefore

10cos 75.cos 15 = 5/2