Revision notes for CIE IGCSE Maths The Magnitude of a Vector. Open the guide for explanations and worked examples. Written against the CIE IGCSE Maths (0580) specification, so the content matches what's examinable rather than general Maths background.
The Magnitude of a Vector
What you'll learn
How to read a column vector as horizontal and vertical movement.
How to find the magnitude, or length, of a vector using Pythagoras.
How to combine vectors in triangles, parallelograms and trapezia before finding a magnitude.
How to give exact answers using square roots and simplified surds.
Vectors as movements
A vector describes a movement with both size and direction. In geometry, AB⃗\vec{AB}AB means “the vector from A to B”. The order of the letters matters.
Definition
Column vector and components
A column vector is a vector written in the form
(xy)\begin{pmatrix} x \\ y \end{pmatrix}(xy)
where the top number, xxx, is the horizontal component and the bottom number, yyy, is the vertical component.
CB⃗\vec{CB}CB and BC⃗\vec{BC}BC have the same length, but they point in opposite directions. Always change both signs when reversing a vector.
Magnitude means length
Definition
Magnitude of a vector
The magnitude of a vector is its length. The magnitude of v⃗\vec{v}v is written as ∣v⃗∣|\vec{v}|∣v∣.
For a column vector v⃗=(xy)\vec{v}=\begin{pmatrix} x \\ y \end{pmatrix}v=(xy), the horizontal and vertical components form a right-angled triangle. So we use Pythagoras:
∣v⃗∣=x2+y2|\vec{v}|=\sqrt{x^2+y^2}∣v∣=x2+y2
Key Idea
The magnitude formula
To find the length of (xy)\begin{pmatrix} x \\ y \end{pmatrix}(xy), square both components, add them, then square root.
Subtract top from top, and bottom from bottom. Keep the order exactly as written.
Trapezia and scalar multiples
Definition
Trapezium and scalar multiple
A trapezium is a quadrilateral with at least one pair of parallel sides. A scalar multiple means a vector has been multiplied by a number, such as 3AB⃗3\vec{AB}3AB.
If you are told DC⃗=3AB⃗\vec{DC}=3\vec{AB}DC=3AB, then DC⃗\vec{DC}DC points in the same direction as AB⃗\vec{AB}AB and is three times as long.
To solve these questions, build the missing side carefully using vector paths.