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Vectors proof questions turn a diagram into algebra. The first job is to write important position vectors from the origin, usually in terms of aaa and bbb.
A side vector is found by subtracting position vectors in the correct order. The key route fact is
AB→=OB→−OA→ \overrightarrow{AB}=\overrightarrow{OB}-\overrightarrow{OA} AB=OB−OASo if OA→=4a\overrightarrow{OA}=4aOA=4a and OB→=3b\overrightarrow{OB}=3bOB=3b, then
AB→=3b−4a,BA→=4a−3b \overrightarrow{AB}=3b-4a,\qquad \overrightarrow{BA}=4a-3b AB=3b−4a,BA=4a−3bReversing the direction changes the sign, so order always matters.
Question 1
4 marksP P\,P is the point on AB AB\,AB such that AP:PB=3:2AP:PB = 3:2AP:PB=3:2
OP→=k(2a+3b) \overrightarrow{OP} = k(2\mathbf{a} + 3\mathbf{b}) OP=k(2a+3b)How is the vector AB→\overrightarrow{AB}AB expressed using the position vectors OA→\overrightarrow{OA}OA and OB→\overrightarrow{OB}OB?
Revision notes for Edexcel GCSE Maths Vectors Proof Questions: explanations and worked examples.