נושא : שימוש בזהויות c o s α = sin ( 90 + α ) ? sin α = sin ( 180 − α ) cos α = s i n ( 90 − α ) ↓ cos α = sin ( 180 − 90 + α ⏟ s i n α ) cos α = s i n ( 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}}}
