Ответ во вложении. Удачи!
(2-sin(4*a)*ctg(2*a))/sin(4*a)=tg(2*a)
1/(sin(2a)*cos(2a))-cos(2a)/sin(2a) =(1-cos^2(2a))/(sin(2a)*cos(2a))=
<span>=sin^2(2a)/(sin(2a)*cos(2a))=tg(2*a)</span>
5χ+3χ=180
<span>8χ=180</span>
<span>χ=180:8</span>
<span>x=22.5</span>
<span>5·22.5=112.5</span>
<span>3·22.5=67.5</span>
<span>BOC=112.5° ABC=67.5° </span>