Express the following as a sum or difference of two trigonometric… - Vidyadip

Question

Express the following as a sum or difference of two trigonometric functions : $2 \cos 35^{\circ} \cos 75^{\circ}$

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Answer

$2 \cos 35^{\circ} \cos 75^{\circ}$
$\begin{aligned}
& =\cos \left(35^{\circ}+75^{\circ}\right)+\cos \left(35^{\circ}-75^{\circ}\right) \\
& =\cos 110^{\circ}+\cos (-40)^{\circ} \\
& =\cos 110^{\circ}+\cos 40^{\circ} \ldots[\because \cos (-\theta)=\cos \theta]
\end{aligned}$

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