Question

Simplify completely: cos 75˚ cos 10˚ - sin 75˚ sin 10˚?

Answer 1

cos 90

Explanation:

Use trig identity:
cos (a + b) = cos a.cos b - sin a.sin b
In this case:
cos 75.cos 15 - sin 75.sin 15 = cos (75 + 15) = cos 90