sina= \sqrt{1-cos^2a}= \sqrt{1- \frac{4}{5} }= \frac{1}{ \sqrt{5} } \\ \\ tga= \frac{sina}{cosa}= \frac{ \frac{1}{ \sqrt{5} } }{ \frac{2}{ \sqrt{5} } } = \frac{1}{2} " alt="cosa= \frac{2}{ \sqrt{5} } \\ \\ sin^2a+cos^2a=1 => sina= \sqrt{1-cos^2a}= \sqrt{1- \frac{4}{5} }= \frac{1}{ \sqrt{5} } \\ \\ tga= \frac{sina}{cosa}= \frac{ \frac{1}{ \sqrt{5} } }{ \frac{2}{ \sqrt{5} } } = \frac{1}{2} " align="absmiddle" class="latex-formula">