Here's the question you clicked on:
azolotor
The median of a triangle is a vector from a vertex to the midpoint of the opposite side. Show the sum of the medians of a triangle = 0. Answer: The median of side AB is the vector from vertex C to the midpoint of AB. Label this midpoint as P. As usual we write P for the origin vector −−→ OP. −−→ CP = 1 2 (A + B) 1 The midpoint P = ⇒ (B + A) − C. 2 Likewise: −−→ BQ = (A + C) − B and −−→ AR = 1 2 1 2 (B + C)
Why is it -c and not +c?
Geometrically I do not see what is happening when we subtract that