cos β =cos120^(o)=-1/2
cos^2 α +cos^2 β +cos^2 γ =1
(1/2)^2+(-1/2)^2+cos^2 γ =1
cos^2 γ =1/2
cos γ =- sqrt(2)/2
(1/2; -1/2; -sqrt(2)/2) - есть ось.
Обозначим vector{b}
пр_(vector{b}) vector{a}=(vector{a}*vector{b})/|vector{b}|=
=(1*(1/2)+3*(-1/2)-1*(-sqrt(2)/2))/sqrt((1/4)+(1/4)+(1/2))=
=(sqrt(2)-2)/2