נושא : שימוש בזהויותcosα=sin(90+α)?sinα=sin(180−α)cosα=sin(90−α)↓cosα=sin(180−90+α⏟sinα)cosα=sin(90+α){\displaystyle {\begin{aligned}&cos\alpha =\sin(90+\alpha )?\\&\color {blue}\sin \alpha =\sin(180-\alpha )\\&\color {blue}\cos \alpha =sin(90-\alpha )\\&\downarrow \\&\cos \alpha =\sin(180-\underbrace {90+\alpha } _{sin\alpha })\\&\cos \alpha =sin(90+\alpha )\\\end{aligned}}}