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Mathematics · Class 11
Add vectors componentwise (or tip-to-tail / parallelogram). Triangle law: a + b is the third side. Scalar multiply: stretch or shrink, reverse if the scalar is negative. Section formula / midpoint: midpoint of AB is a+b2 if a,b are position vectors. Meaning: addition is 'follow one arrow, then the other'.
PracticeAdd (or subtract) entrywise, only if the sizes match. Scalar k times A multiplies every entry by k. Negative: -A = (-1)A. Meaning: addition is 'same box, add matching numbers'. It is not matrix multiplication (next page).
PracticeAllied angles are built from A by adding or subtracting 90, 180, 270, 360 (and negatives). Examples: sin(180-A)=sin A, cos(180-A)=-cos A, tan(180-A)=-tan A. sin(90-A)=cos A. The CAST (or ASTC) rule names which functions are positive in which quadrant. Meaning: reduce to an acute related angle, then attach the right sign.
PracticeAngle between two vectors: use the dot (cos) unless the question asks for sin (then |cross|). Angle between a vector and a plane, or two planes, is a Grade 12-style extra — only if the book asks. Work W = F · d. Area of triangle ABC is (12)|AB x AC|. Meaning: pick dot for 'how aligned', pick cross for 'how much area / a perpendicular'.