18arccos1/2 - 2arcsin (-1/2)+ arstg √3

18arccos1/2=18cos^-1(1/2)=6п;2arcsin (-1/2)=2*sin^-1(-1/2)=-п/3;arstg √3=tg^-1(√3)=п/3;6п-(-п/3)+(п/3)=20п/3