## Trigonometry 7th Edition

$2 \sqrt 2$
Given expression = 4 $\cos y^{\circ}$ = 4 $\cos 45^{\circ}$ ( replacing y with $45^{\circ}$) = 4 $\times (\frac{1}{\sqrt 2})$ ( substituting for exact value of $\cos 45^{\circ}$) = $2\times\sqrt 2\times\sqrt 2\times (\frac{1}{\sqrt 2})$ ( factorising 4 as $2\times\sqrt 2\times\sqrt 2)$ = $2 \sqrt 2$