a∈ [90°;180°] - 3-я четверть. Косинус в этой четверти отрицательный:
\ cosa=-\frac{4}{5} \\ \\ tga=\frac{sina}{cosa}=\frac{\frac{3}{5}}{-\frac{4}{5}} =-\frac{3}{5}*\frac{5}{4}=-\frac{3}{4} \\ \\ tga+cosa=-\frac{3}{4}-\frac{4}{5} =-(\frac{15}{20} +\frac{16}{20} )=-\frac{31}{20} =-1.55 \\ \\ OTBET: \ -1.55 " alt=" cos^2a=1-sin^2a=1-(\frac{3}{5})^2=1-\frac{9}{25} =\frac{16}{25} \\ \\ cosa=^+_-\sqrt{\frac{16}{25}} =^+_-\frac{4}{5} \ => \ cosa=-\frac{4}{5} \\ \\ tga=\frac{sina}{cosa}=\frac{\frac{3}{5}}{-\frac{4}{5}} =-\frac{3}{5}*\frac{5}{4}=-\frac{3}{4} \\ \\ tga+cosa=-\frac{3}{4}-\frac{4}{5} =-(\frac{15}{20} +\frac{16}{20} )=-\frac{31}{20} =-1.55 \\ \\ OTBET: \ -1.55 " align="absmiddle" class="latex-formula">