Умоляю, помогите с алгеброй, я дерево

Решите задачу:

$\dfrac{sin \dfrac{12\pi}{5}-3sin \dfrac{3 \pi }{5} } {3cos \dfrac{\pi}{10}+5cos \dfrac{11\pi}{10} }= \\ = \dfrac{sin \dfrac{2\pi}{5}-3sin \dfrac{3 \pi }{5} } {3cos \dfrac{\pi}{10}+5cos \dfrac{11\pi}{10} }= \\ = \dfrac{-2sin \dfrac{\pi}{10}cos \dfrac{\pi}{2}-2sin \dfrac{3\pi}{5} }{6cos \dfrac{3\pi}{5}cos \dfrac{\pi}{2}+2cos \dfrac{11\pi}{10} } = \\ =\dfrac{-sin \dfrac{3\pi}{5} }{cos \dfrac{11\pi}{10} } = \\ =- \dfrac{sin \dfrac{3\pi}{5} }{cos \left( \dfrac{ \pi}{2}+ \dfrac{3 \pi }{5}\right)}=$
$=1$