10 класс алгебра !! 128(Б)!

Решите задачу:

$\sin( \alpha - \beta ) =\sin \alpha \cos \beta +\sin \beta \cos \alpha ,\\\sin(- \alpha )=-\sin \alpha ,\ \ \cos(- \beta )=\cos \beta . \\\\\sin{\left( \alpha - \beta \right)}+\sin{\left(- \alpha \right)}\cos{\left(- \beta \right)}=\sin{\left( \alpha - \beta \right)}+\left(-\sin{\alpha}\cos{\beta}\right)=\\=\sin \alpha \cos \beta +\sin \beta \cos \alpha -\sin{\alpha}\cos{\beta}=\sin \beta \cos \alpha .$