Ctgα=√2-1 cos α/ sin α=√2-1 cos a=sin α*(√2-1) cos α=√(1-cos^2 α)*(√2-1) cos^2α=(1-cos^2 α)*(2-2√2+1) cos^2α=(1-cos^2 α)*(3-2√2) cos^2α(1+3-2√2)=3-2√2 cos^2 α(4-2√2)=3-2√2 cos α=√((3-2√2)/(4-2√2)) α=67.5+2p*k cos(67.5*2)=-√2/2