Derive Difference of Two Angles Identities (Complex Plane)

This example demonstrates how the difference of two angles identities can be derived using the properties of complex numbers and Euler’s formula. The two difference identities are shown below:

Steps

  1. Observe that the angle (theta) forms the complex number on the unit circle given by Euler’s formula.

    This complex number is visualized by the figure below:

    A point on the unit circle in the complex plane.
    Figure 1: Complex Unit Circle
  2. Model the difference of the two angles (alpha) and (beta) as input to the exponential function.

    Distribute the complex constant for both angles.

  3. Apply the addition property of the exponential function to expand to right-hand side.

  4. Apply Euler’s formula to both sides of the equation where the exponential function appears. Note, when applying Euler’s formula for the negative angle the following symmetry can be observed.

    A point on the unit circle in the complex plane corresponding to a negative angle.
    Figure 2: Complex Unit Circle Negative Angle

    This results in the following expression.

    Multiply the right-hand side of the equation.

    Substitute for in the expression.

    Simplify and group the real and complex parts of the right-hand side.

  5. Equate the real parts of the complex number and the complex parts of the number.

    Divide both sides of the second expression by to finish deriving the two summation identities.

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