What is the value of cos (15°) + cos (105°) ?

\(\frac{1}{\sqrt {2 }}\) \(\frac{2\sqrt {3 } + 1}{2}\) \(\sqrt {3 }\) \(\frac{1}{\sqrt {3 }}\)

\(\frac{1}{\sqrt {2 }}\)

⇒ cos (15°) + cos (105°) = 2cos (\(\frac{120}{2}\)) cos (\(\frac{90}{2}\)) ( because cos A + cos B = 2 cos (\(\frac{A+B}{2}\)) cos (\(\frac{A-B}{2}\)) = 2cos 60° cos 45° = 2 × \(\frac{1}{2}\) × \(\frac{1}{\sqrt {2 }}\) = \(\frac{1}{\sqrt {2 }}\)