$\cos \left(7 \frac{1}{2}\right)^{\circ}=?$

$\sqrt{2+\sqrt{2+\sqrt{3}}}$

$\frac{1}{2} \sqrt{2+\sqrt{2+\sqrt{3}}}$

$\frac{1}{2} \sqrt{2-\sqrt{2+\sqrt{3}}}$

$\sqrt{2-\sqrt{2+\sqrt{3}}}$