Q.Define coinitial vectors.

Coinitial Vectors

Stand at one spot in a field and give two instructions: "walk 5 steps north" and "walk 3 steps east." Both journeys begin at the same place — your feet. That shared starting point is the whole idea behind coinitial vectors.

Every vector has a tail (its initial point, where it starts) and a tip (its terminal point). When two or more vectors share the same tail, they are called coinitial vectors.

Precise definition

Vectors $\vec{a}$ and $\vec{b}$ are coinitial if they have a common initial point. If both are drawn starting from a point $O$, they are coinitial — no matter where their tips land. Their magnitudes and directions may be completely different; only the starting point must coincide.

Why it matters …