---
title: "Euler's Formula and the Exponential Form"
description: "e to the i theta traces the unit circle, tying the exponential function to sine and cosine. In exponential form, multiplying complex numbers means adding angles, and De Moivre's theorem falls out for "
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retrieved: 2026-09-12
---

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# Euler's Formula and the Exponential Form

e to the i theta traces the unit circle, tying the exponential function to sine and cosine. In exponential form, multiplying complex numbers means adding angles, and De Moivre's theorem falls out for free.

Subject: Mathematics · Area: Algebra · Ages 17 to 18
Page: https://lightmysky.com/learn/mathematics/eulers-formula-and-the-exponential-form-mt_RX2IxsnSwG

## Ready when they can

- Write a complex number in exponential form and convert back
- Multiply and divide complex numbers by adding and subtracting arguments
- Use De Moivre's theorem to compute a small integer power of a complex number

## Lesson: Angles that multiply

Euler packs direction into an exponential: e to the iT equals cos T plus i sin T. As T grows, the value traces the unit circle, so its modulus is always 1. Landmarks fall out free: e to the i pi over 2 is i, and e to the i pi is minus one.

**Example.** Multiplying splits into two easy jobs: multiply the moduli, add the arguments. So 2e to the i pi over 6 times 3e to the i pi over 3 is 6e to the i pi over 2, which is 6i. Division mirrors it: 2e to the i pi over 4 over 4e to the i pi over 4 is 0.5, since the arguments cancel and 2 over 4 is 0.5.

De Moivre extends the trick to powers: raise the modulus to n and multiply the angle by n. Squaring cos 30 plus i sin 30 gives cos 60 plus i sin 60, and cubing cos 20 plus i sin 20 gives cos 60 plus i sin 60. Even (1 + i) to the 4th surrenders: modulus root 2 to the 4th is 4, angle 4 times 45 is 180 degrees, so the answer is minus 4.

**Tip.** Pick the form per task and convert freely. Add and subtract in Cartesian, multiply and divide in exponential. A conjugate simply negates the argument, and answers expand back with x equals r cos T and y equals r sin T.

**Recap.** Exponential form turns multiplication into adding angles, and De Moivre turns powers into multiplying them.

## Practice

18 questions on this page, each with its working shown.

## Needs first

- [The Exponential Function and the Number e](https://lightmysky.com/learn/mathematics/the-exponential-function-and-the-number-e-mt_2hyjN4JfJJ)
- [The Argand Plane, Modulus and Argument](https://lightmysky.com/learn/mathematics/the-argand-plane-modulus-and-argument-mt_Wq01c5UN9e)

## Opens up

- [Second-Order Linear Equations with Constant Coefficients](https://lightmysky.com/learn/mathematics/second-order-linear-equations-with-constant-coefficients-mt_G7sMq1EHwQ)
- [Impedance and Phasors in AC Circuits](https://lightmysky.com/learn/science/impedance-and-phasors-in-ac-circuits-mt_vQVlVWv2ln)
- [Roots of Polynomials and Roots of Unity](https://lightmysky.com/learn/mathematics/roots-of-polynomials-and-roots-of-unity-mt_XAISygqdQr)