Ctg(1/2 arccos (-1/3))
По формулу ctg a/2=sina/(1-cosa) получим ctg(1/2arccos(-1/3))= sin(arccos(-1/3)/(1-cos(arccos(-1/3) )= sin(π-arccos1/3)/(1-cos(arccos-1/3))=sin(arccos1/3)/(1-cos(π-arccos1/3)=√1-1/9 /(1+1/3)=√8/9/4/3=2/3√2/4/3=1/2√2.