треугольника АВС, если A(1, 2, 1), B(3, 1, 1), C(0, 2, 1).
A(1, 2, 1), B(3, -1, 1), C(0, 2, -1).
vector{AC}=(0-1; 2-2; -1-1)=(-1; 0; -2)
vector{AВ}=(3-1; -1-2; 1-1)=(2; -3; 0)
vector{AC}*vector{AB}=|vector{AC}|*|vector{AB}|*cos ∠ A
vector{AC}*vector{AB}=-1*(-2)+0*(-3)+(-2)*0=2
|vector{AC}|=sqrt((-1)^2+0^2+(-2)^2)=sqrt(5)
|vector{AB}|=sqrt(2^2+(-3)^2+0^2)=sqrt(13)
cos ∠ A=vector{AC}*vector{AB}/(|vector{AC}|*|vector{AB}|)=
=2/(sqrt(13)*sqrt(5))=2/sqrt(65)