Question

How do you find the exact functional value cos25°cos15° - sin25°sin15° using the cosine sum or difference identity?

Answer 1

Exact value of cos 25cos 15 - sin 25.sin 15.

Ans: 0.766

Explanation:

Use the trig identity: cos (a + b) = cos a,cos b - sin a.sin b
cos (25 + 15) = cos (40) = cos 25.cos 15 - sin 25.sin 15.
The arc `40^@` doesn't have exact functional value.
You may get its approx. value by calculator.
cos 40 = 0,766