Chapter 7 - Trigonometric Identities and Equations - 7.4 Double-Angle and Half-Angle Identities - 7.4 Exercises - Page 693: 29

$\dfrac {\cos 94.2}{4}$

$1-2\sin ^{2}\alpha =\cos 2\alpha \Rightarrow \dfrac {1}{4}-\dfrac {1}{2}\sin ^{2}47.1=\dfrac {1}{4}\left( 1-2\sin ^{2}47.1\right) =\dfrac {1}{4}\cos \left( 2\times 47.1\right) =\dfrac {\cos 94.2}{4}$