Cos=-4/5 180 найдите cos2a

[latex]cos2a=cos ^{2} a-sin ^{2} s=cos ^{2} a- \\ -(1-cos ^{2} a)=cos ^{2} a-1+cos ^{2} a=\\=2cos ^{2} a-1 \\ \\ cosa=- \frac{4}{5} \\ \\ 2*(- \frac{4}{5} ) ^{2} -1=2* \frac{16}{25} -1= \frac{32}{25} - \frac{25}{25} =\\\\= \frac{7}{25} = \frac{7*4}{25*4} = \frac{28}{100} =0.28[/latex]