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Geometry & measures · Vectors

Chapter 1 · 3

The idea

Vectors on a shape

Expressing any journey around a figure in terms of two given vectors — why every route gives the same answer, why going against an arrow flips the sign, and why a point splitting a side in the ratio 2:3 sits two fifths along, not two thirds.

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Geometry & measures · Vectors

Vectors on a shape

Expressing any journey around a figure in terms of two given vectors — why every route gives the same answer, why going against an arrow flips the sign, and why a point splitting a side in the ratio 2:3 sits two fifths along, not two thirds.

Why it works

The same vector, anywhere on the shape

A vector is a journey, not a place. It records two things and only two things — how far, and in which direction. It records nothing about where the journey started. That one fact is the engine of this whole topic, because it means the same vector can appear in several different places on the same diagram. In a parallelogram OABCOABC with OA→=a\overrightarrow{OA} = \mathbf{a}, the opposite side CBCB is the same length as OAOA and points the same way, so CB→=a\overrightarrow{CB} = \mathbf{a} as well. Not "something like a\mathbf{a}" — the very same vector, written with the very same letter.

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