Your History

Menu

Euler Definition of Cosine

Prerequisites

Cosine is an even function | \(\cos(\theta) = \cos(-\theta)\)
Sine is an odd function | \(\sin(-\theta) = -\sin(\theta)\)
Eulers Formula | \(e^{j \cdot \theta} = \cos(\theta) + j \sin(\theta)\)

Description

This equation defines the csine function (\( \htmlClass{sdt-0000000124}{\cos} \)) for an angle (\( \htmlClass{sdt-0000000024}{\theta} \)) in terms of eulers constant (\( \htmlClass{sdt-0000000035}{e} \)) and the imaginary unit (\( \htmlClass{sdt-0000000129}{j} \)).

\[\htmlClass{sdt-0000000124}{\cos}(\htmlClass{sdt-0000000024}{\theta}) = \frac{\htmlClass{sdt-0000000035}{e}^{\htmlClass{sdt-0000000129}{j} \cdot \htmlClass{sdt-0000000024}{\theta}} + \htmlClass{sdt-0000000035}{e}^{-\htmlClass{sdt-0000000129}{j} \cdot \htmlClass{sdt-0000000024}{\theta}}}{2}\]

Symbols Used:

This is a commonly used symbol to represent an angle in mathematics and physics.
\( e \)

This symbol represents Euler's constant. It is approximately \(2.718\).
\( \cos \)

This is the symbol for cosine, a trigonometric function that calculates the ratio of the adjacent side to the hypotenuse of a right-angled triangle.
\( j \)

This symbol represents the imaginary unit, which is defined as the square root of \(-1\). \( j = \sqrt{-1}\). It is the most fundamental unit in the field of complex numbers, allowing for the expression of numbers that cannot be represented on the real number line.

Derivation

  1. Consider Euler's Formula:
    \[\htmlClass{sdt-0000000035}{e}^{\htmlClass{sdt-0000000129}{j} \cdot \htmlClass{sdt-0000000024}{\theta}} = \htmlClass{sdt-0000000124}{\cos}(\htmlClass{sdt-0000000024}{\theta}) + \htmlClass{sdt-0000000129}{j} \htmlClass{sdt-0000000127}{\sin}(\htmlClass{sdt-0000000024}{\theta})\]
  2. Let us also consider a version of Euler's Formula where we use \(-\htmlClass{sdt-0000000024}{\theta}\) instead:
    \[\htmlClass{sdt-0000000035}{e}^{\htmlClass{sdt-0000000129}{j} \cdot - \htmlClass{sdt-0000000024}{\theta}} = \htmlClass{sdt-0000000124}{\cos}(-\htmlClass{sdt-0000000024}{\theta}) + \htmlClass{sdt-0000000129}{j} \cdot \htmlClass{sdt-0000000127}{\sin}(\htmlClass{sdt-0000000024}{\theta})\]
  3. We can now simplify using the fact that sine is an odd function and cosine is an even function:
    \[\htmlClass{sdt-0000000127}{\sin}(-\htmlClass{sdt-0000000024}{\theta}) = -\htmlClass{sdt-0000000127}{\sin}(\htmlClass{sdt-0000000024}{\theta})\]
    \[\htmlClass{sdt-0000000124}{\cos}(\htmlClass{sdt-0000000024}{\theta}) = \htmlClass{sdt-0000000124}{\cos}(-\htmlClass{sdt-0000000024}{\theta})\]
    to get...
    \[\htmlClass{sdt-0000000035}{e}^{-\htmlClass{sdt-0000000129}{j} \cdot \htmlClass{sdt-0000000024}{\theta}} = \htmlClass{sdt-0000000124}{\cos}(\htmlClass{sdt-0000000024}{\theta}) - \htmlClass{sdt-0000000129}{j} \cdot \htmlClass{sdt-0000000127}{\sin}(\htmlClass{sdt-0000000024}{\theta})\]
  4. We can now add this to Euler's Formula (see step (1)) to get
    \[\htmlClass{sdt-0000000035}{e}^{\htmlClass{sdt-0000000129}{j} \cdot \htmlClass{sdt-0000000024}{\theta}} + \htmlClass{sdt-0000000035}{e}^{-\htmlClass{sdt-0000000129}{j} \cdot \htmlClass{sdt-0000000024}{\theta}} = 2 \cdot \htmlClass{sdt-0000000124}{\cos}(\htmlClass{sdt-0000000024}{\theta})\]
  5. We can now divide both sides by \(2\) to get...
    \[\htmlClass{sdt-0000000124}{\cos}(\htmlClass{sdt-0000000024}{\theta}) = \frac{\htmlClass{sdt-0000000035}{e}^{\htmlClass{sdt-0000000129}{j} \cdot \htmlClass{sdt-0000000024}{\theta}} + \htmlClass{sdt-0000000035}{e}^{-\htmlClass{sdt-0000000129}{j} \cdot \htmlClass{sdt-0000000024}{\theta}}}{2}\]

Example

Coming soon...