Legea fundamentala a trigonometriei
[tex]sin^2{x}+cos^2{x}=1\\= > cos(x)=-\sqrt{1-sin^2{x}}, daca \;\; x \in (\frac{\pi}{2};\frac{3\pi}{2})\\= > cos(x)=\sqrt{1-sin^2{x}}, daca \;\; x \in [0;\frac{\pi}{2}] \cup [\frac{3\pi}{2};2*\pi][/tex]
x unghi ascutit => x apartine [0;pi/2]
[tex]cos(x) = \sqrt{1 - sin^2{x}}=\sqrt{1-\frac{16}{25}}=\sqrt{\frac{25-16}{25}}=\sqrt{\frac{9}{25}}=\frac{3}{5}[/tex]
