[tex]sin\alpha = \frac{3}{5}[/tex]
Korzystamy z tzw. jedynki trygonometrycznej:
sin²α + cos²α = 1 ⇒ cos²α = 1 - sin²α
[tex]cos^{2}\alpha = 1 - sin^{2}\alpha = 1-(\frac{3}{5})^{2} = \frac{25}{25}-\frac{9}{25} = \frac{16}{25}\\\\cos\alpha = \sqrt{\frac{16}{25}} = \frac{\sqrt{16}}{\sqrt{25}}\\\\\boxed{cos\alpha = \frac{4}{5}}[/tex]