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$\frac{-2sin(\frac{13}{2}\alpha)sin(-\frac{\alpha}{2})-2sin(\frac{17}{2}\alpha)sin(\frac{\alpha}{2})}{2cos(\frac{13}{2}\alpha)sin(-\frac{\alpha}{2})+2cos(\frac{17}{2}\alpha)sin(\frac{\alpha}{2})}$ = $\frac{sin(\frac{13}{2}\alpha)sin(\frac{\alpha}{2})-sin(\frac{17}{2}\alpha)sin(\frac{\alpha}{2})}{cos(\frac{17}{2}\alpha)sin(\frac{\alpha}{2})-cos(\frac{13}{2}\alpha)sin(\frac{\alpha}{2})}=\frac{sin(\frac{13}{2}\alpha)-sin(\frac{17}{2}\alpha)}{cos(\frac{17}{2}\alpha)-cos(\frac{13}{2}\alpha)}$ = $\frac{-2cos(\frac{15}{2}\alpha)sin\alpha}{-2sin(\frac{15}{2}\alpha)sin\alpha}=ctg(\frac{15}{2}\alpha)$