α - угол первой четверти , значит Sinα > 0 .
[tex]\displaystyle\bf\\Cos2\alpha =0,98\\\\Cos2\alpha =1-2Sin^{2} \alpha\\\\2Sin^{2} \alpha =1-Cos2\alpha =1-0,98=0,02\\\\Sin^{2} \alpha =0,02:2=0,01\\\\Sin\alpha =\sqrt{0,01} =0,1\\\\Otvet: \ Sin\alpha =0,1[/tex]
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α - угол первой четверти , значит Sinα > 0 .
[tex]\displaystyle\bf\\Cos2\alpha =0,98\\\\Cos2\alpha =1-2Sin^{2} \alpha\\\\2Sin^{2} \alpha =1-Cos2\alpha =1-0,98=0,02\\\\Sin^{2} \alpha =0,02:2=0,01\\\\Sin\alpha =\sqrt{0,01} =0,1\\\\Otvet: \ Sin\alpha =0,1[/tex]