Срочно)!!!! вычислить tg(0.5 arccos (-4/17))

Решите задачу:

$tg(\frac{1}{2}arccos(-\frac{4}{17}))=?\\\\tg\frac{ \alpha }{2}= \frac{sin \alpha }{1+cos \alpha } \; \; ,\; \; \alpha =arccos(-\frac{4}{17})\\\\arccos(-\frac{4}{17})=\pi -arccos\frac{4}{17}\; \; \Rightarrow \\\\\\sin(arccos(-\frac{4}{17}))=sin(\pi -arccos\frac{4}{17})=sin(arccos\frac{4}{17})=\\\\=\sqrt{1-cos^2(arccos\frac{4}{17})}=\sqrt{1-( \frac{4}{17} )^2}=\sqrt{1- \frac{16}{289} }=\frac{\sqrt{273}}{17}\\\\\\cos(arccos(-\frac{4}{17}))=cos(\pi -arccos\frac{4}{17})=-cos(arccos\frac{4}{17})=-\frac{4}{17}$

$tg(\frac{1}{2}arctg(-\frac{4}{17}))=tg \frac{ \alpha }{2} = \frac{\frac{\sqrt{273}}{17}}{1-\frac{4}{17}} = \frac{\sqrt{273}\cdot 17}{17\cdot 13} = \frac{\sqrt{273}}{17}$