Complex Number Multiplication
Lesson notes
Overview
Multiplying complex numbers multiplies their lengths and adds their angles. This topic is rated intermediate and is number 56 of 245 in Mathematics. Builds on the basics with relationships, methods and trade-offs.
What you are seeing
The panel above shows an interactive animation of the idea. Every slider changes one quantity so you can see its effect on the outcome. The animation is drawn live from the model, so everything you see responds to the sliders. Press Space to pause, use Restart to replay from the start, and change the speed to slow down quick events.
In depth
To multiply two complex numbers, multiply their lengths (moduli) and add their angles (arguments). So multiplication by a complex number is a combined stretch and rotation of the plane. Multiplying by i turns a point 90° anticlockwise. This geometric view explains why i² = −1, since two quarter-turns make a half-turn, and why complex numbers are so useful for describing rotations and oscillations.
Reading the relationship
The expression "|z₁z₂| = |z₁||z₂|" is a compact summary of the idea. It uses the symbols z₁z, z₁, z₂; name what each one stands for before you start. Then use the sliders to see how changing one quantity changes the others.
A guided exploration
1. Run the animation once with the default settings and describe in words what happens. 2. Change one slider at a time and note which quantity responds most. 3. Push each slider to its extreme and decide whether the result still makes sense.
Try this
Predict the outcome first, then test extreme slider values to find where the pattern changes or breaks.
Common pitfalls
A frequent slip is memorising the rule without the picture. If a result surprises you, sketch the graph or test a simple number by hand before trusting the algebra.
How it connects
It builds on “Euler’s Formula”, the topic before it in Mathematics. Once you are comfortable, continue with “Roots of Unity”.
Why it matters
Mathematical ideas like this one are the language that physics, engineering, computing and statistics are written in.
Where you might use it
- Revising “Complex Number Multiplication” before a class, exam or certification
- Checking working on homework and exam questions by watching the graph respond
- Modelling growth, change and uncertainty in finance, data science and statistics
- Building the reasoning behind the algorithms and engineering tools people use every day
Frequently asked questions
What is Complex Number Multiplication?
Multiplying complex numbers multiplies their lengths and adds their angles. It is an intermediate topic in Mathematics.
How do I use the Complex Number Multiplication simulation?
The panel at the top shows an interactive animation of the idea. Every slider changes one quantity so you can see its effect on the outcome. Move the sliders one at a time, then press Space to pause the animation and look closer.
What does the formula mean?
The expression |z₁z₂| = |z₁||z₂| packs the idea into symbols. Name each symbol first, then change one quantity with the sliders and see which others respond.
Who is this topic for?
Builds on the basics with relationships, methods and trade-offs. If the quiz feels hard, reread the lesson notes and try the previous topic first.
What should I learn before this?
Start with “Euler’s Formula”, the previous topic in Mathematics. Each topic in a subject builds on the ones before it.
How do I track my progress?
Press “Mark as learned” on this page. Progress is stored only in this browser and is never sent anywhere. The Browse page can show topics you have learned or still need to learn.
Quick quiz
More in Mathematics
See all →Roots of Unity
The n solutions of zⁿ = 1 are the corners of a regular n-gon on the unit circle.
The Derivative as a Slope
Zoom in on a curve: the secant line settles into the tangent, whose slope is the derivative.
Tangent of a Sine Wave
The slope of sin x at each point is cos x — watch the secant rotate as it slides.