Cos105cos5+sin105cos85/sin95cos5-cos95sin185. почему получается -tg70? объясните

= (Cos105cos5+sin105cos(90-5)) /(sin95cos5-cos95sin(180+5))=
={Cos105cos5+sin105sin5) / (sin95cos5+cos95sin5)=
=cos(105-5)/sin(95+5)=
=cos100/sin100=ctg100=ctg(90+10)=
=-tg10.