\sin\alpha=\frac{2}{3}
\alpha \subset(0,\pi)
sin^2\alpha+cos^2\alpha=1
(\frac{2}{3})^2+cos^2\alpha=1
\frac{4}{9}+cos^2\alpha=1
cos^2\alpha=1-\frac{4}{9}=\frac{5}{9}
cos\alpha=\frac{\sqrt{5}}{3}
lub
cos\alpha=-\frac{\sqrt{5}}{3}
sin\alpha*cos\alpha=\frac{2}{3}*\frac{\sqrt{5}}{3}=\frac{2\sqrt{5}}{9}
lub
sin\alpha*cos\alpha=\frac{2}{3}*(-\frac{\sqrt{5}}{3})=-\frac{2\sqrt{5}}{9}