the exact vaule of cos(247.5 degrees)
\(cos(247.5 deg)=cos( \frac{495deg}{2}) = - \sqrt{ \frac{1+cos(495deg)}{2} } = -\sqrt{ \frac{1- \frac{ \sqrt{2} }{2} }{2} } = \frac{ \sqrt{2- \sqrt{2} } }{2}\)
