If the vectors i + 2 j + 3 k , λ i + 4 j + 7 k , - 3 i - 2 j - 5 k are collinear if λ equals
Recall the rule after simplifying, 3 is left. The method is simple: formula → substitution → calculation → answer.
Ref: NCERT Mathematics Class XII — Vectors and 3D Geometry
