А(0;1;-1) и С(3;1;0)
vector{BC}=(3-1;1-(-1);0-2)=(2;2;-2)
vector{BA}*\vector{BC}=|vector{BA}|*|vector{BC}|* cos ∠ B
vector{BA}*vector{BC}=-1*2+2*2+(-3)*(-2)=-2+4+6=8
|vector{BA}|=sqrt((-1)^2+2^2+(-3)^2)=sqrt(14)
|vector{BC}|=sqrt(2^2+2^2+(-2)^2)=sqrt(12)=2sqrt(3)
cos ∠ B=8/(sqrt(14)*2sqrt(3))=4/sqrt(42)=4sqrt(42)/42=[b]2sqrt(42)/21
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