Solution :
The value of cos 15 degrees is \(\sqrt{3} + 1\over 2\sqrt{2}\).
Proof :
We will write cos 15 as cos (45 – 30).
By using formula cos (A – B) = cos A cos B + sin A sin B,
cos (45 – 30) = cos 45 cos 30 + sin 45 sin 30
\(\implies\) cos 15 = \(1\over \sqrt{2}\) \(\times\) \(\sqrt{3}\over 2\) + \(1\over \sqrt{2}\) \(\times\) \(1\over 2\)
\(\implies\) cos 15 = \(\sqrt{3} + 1\over 2\sqrt{2}\)
