Explanation:
To find a vector between two points, trace a known path along the diagram, reversing the sign of a vector when travelling against its arrow (e.g. AB = AO + OB = −a + b = b − a). If a point divides a line in the ratio m : n, use the fraction mm + n of the full vector.
To prove two vectors are parallel, factorise both so they are scalar multiples of the same bracketed vector (e.g. PQ = k × RS). If they also share a common point (such as P, Q, R where PQ = k × QR), then the three points are collinear (lie on a straight line).
Example 1: In ΔOAB, OA = a and OB = b. M is the midpoint of OA and N divides OB in the ratio 1 : 2. Find MN in terms of a and b.
Solution:
MO = −12a; ON = 13b (ratio 1 : 2)
MN = MO + ON ⇒ Answer: 13b − 12a
Example 2: Given PQ = 2a + 4b and QR = 3a + 6b, prove that P, Q, and R lie on a straight line.
Solution:
Factorise both vectors:
PQ = 2(a + 2b) and QR = 3(a + 2b)
Therefore QR = 32PQ, so PQ and QR are parallel
Since they also share the common point Q, P, Q, and R are collinear.