x - угол четвёртой четверти , значит Cosx >0 , tgx < 0 .
[tex]\displaystyle\bf\\Sinx=-0,8\\\\Cosx=\sqrt{1-Sin^{2} x} =\sqrt{1-(-0,8)^{2} } =\sqrt{1-0,64} =\sqrt{0,36} =0,6\\\\\boxed{Cosx=0,6}\\\\\\tgx=\frac{Sinx}{Cosx} =\frac{-0,8}{0,6} =-1\frac{1}{3} \\\\\\\boxed{tgx=-1\frac{1}{3}} \\\\\\Cos2x=1-2Sin^{2} x=1-2\cdot(-0,8)^{2}=1-2\cdot 0,64=1-1,28=-0,28\\\\\boxed{Cos2x=-0,28}[/tex]
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x - угол четвёртой четверти , значит Cosx >0 , tgx < 0 .
[tex]\displaystyle\bf\\Sinx=-0,8\\\\Cosx=\sqrt{1-Sin^{2} x} =\sqrt{1-(-0,8)^{2} } =\sqrt{1-0,64} =\sqrt{0,36} =0,6\\\\\boxed{Cosx=0,6}\\\\\\tgx=\frac{Sinx}{Cosx} =\frac{-0,8}{0,6} =-1\frac{1}{3} \\\\\\\boxed{tgx=-1\frac{1}{3}} \\\\\\Cos2x=1-2Sin^{2} x=1-2\cdot(-0,8)^{2}=1-2\cdot 0,64=1-1,28=-0,28\\\\\boxed{Cos2x=-0,28}[/tex]