Answer
$2 \sqrt 2$
Work Step by Step
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)$
= $2 \sqrt 2$
