נושא : שימוש בזהויות s i n α = cos ( 90 + α ) ? sin α = cos ( 90 − α ) cos α = − c o s ( 180 − α ) ↓ cos α = − c o s ( 180 − α ) cos α = − c o s [ 180 − ( 90 − α ) ⏟ s i n α = c o s ( 90 − α ) ] c o s α = − c o s ( 90 + α ) ↓ sin α = − cos ( 90 + α ) {\displaystyle {\begin{aligned}&sin\alpha =\cos(90+\alpha )?\\&\color {blue}\sin \alpha =\cos(90-\alpha )\\&\color {blue}\cos \alpha =-cos(180-\alpha )\\&\downarrow \\&\color {blue}\cos \alpha =-cos(180-\alpha )\\&\cos \alpha =-cos[180-\underbrace {(90-\alpha )} _{\color {blue}sin\alpha =cos(90-\alpha )}]\\&cos\alpha =-cos(90+\alpha )\\&\downarrow \\&\color {blue}\sin \alpha =-\cos(90+\alpha )\\\end{aligned}}}
![{\displaystyle {\begin{aligned}&sin\alpha =\cos(90+\alpha )?\\&\color {blue}\sin \alpha =\cos(90-\alpha )\\&\color {blue}\cos \alpha =-cos(180-\alpha )\\&\downarrow \\&\color {blue}\cos \alpha =-cos(180-\alpha )\\&\cos \alpha =-cos[180-\underbrace {(90-\alpha )} _{\color {blue}sin\alpha =cos(90-\alpha )}]\\&cos\alpha =-cos(90+\alpha )\\&\downarrow \\&\color {blue}\sin \alpha =-\cos(90+\alpha )\\\end{aligned}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/23346298dae2fc6a3157d7f9cba2789075587a86)
