I came across a student asking for help in EduSnap, a mobile app which let students have the chance to post their questions and kind souls answer.
This is the question on vectors (7iii)
and one man named “h2mathtutor” provided a WRONG answer!
There are 2 mistakes:
1) The labelling of vertices should be in order (anticlockwise or clockwise),
2) The working should use cross product instead of dot product. However using cross product is redundant because one already uses it in (ii). Hence a smart student should link (iii) with (ii). I.e. the perpendicular distance from C to AB is the perpendicular height of the parallelogram. Hence the answer is area of parallelogram in (ii) divide by the length AB (magnitude of vector AB).