sin^2\alpha+cos^2\alpha=1
sin\alpha=\frac{3}{4}
(\frac{3}{4})^2+cos^2\alpha=1
\frac{9}{16}+cos^2=1
cos^2\alpha=1-\frac{9}{16}
cos\alpha=\sqrt{\frac{7}{16}}=\frac{\sqrt7}{4}
…
tg\alpha=\frac{sin\alpha}{cos\alpha}=\frac{\frac{3}{4}}{\frac{\sqrt7}{4}}=\frac{3}{4}*\frac{4}{\sqrt7}=\frac{3}{\sqrt7}*\frac{\sqrt7}{\sqrt7}=\frac{3\sqrt7}{7}