Problem 108 - Algebra and Trigonometry Real Mathematics, Real People

Question

Use DeMoivre's Theorem to find the indicated power of the complex number. Write the result in standard form. $$(2+2 i)^{6}$$

Step-by-Step Solution

Answer

The sixth power of the complex number (2+2i) is equal to \(0 - 64i\).

Step 1: Convert to Polar Form

The complex number \(2+2i\) can be converted into polar form using the equations \(r = \sqrt{a^2 + b^2}\) and \(\theta = \arctan{\left(\frac{b}{a}\right)}\), where \(r\) and \(\theta\) are the magnitude and argument of the complex number respectively. For this complex number, \(a = 2\) and \(b = 2\), so \(r = \sqrt{2^2+2^2} = 2\sqrt{2}\) and \(\theta = \arctan{\left(\frac{2}{2}\right)} = \frac{\pi}{4}\). Hence, \(2+2i = 2\sqrt{2}(\cos{\frac{\pi}{4}} + i \sin{\frac{\pi}{4}})\).

Step 2: Apply DeMoivre's Theorem

DeMoivre's Theorem states that \((r(\cos{\theta} + i \sin{\theta}))^n = r^n(\cos{n\theta} + i \sin{n\theta})\). Applying the theorem to our complex number gives \((2\sqrt{2}(\cos{\frac{\pi}{4}} + i \sin{\frac{\pi}{4}}))^6 = (2\sqrt{2})^6(\cos{6 \cdot \frac{\pi}{4}} + i \sin{6 \cdot \frac{\pi}{4}}) = 64(\cos{\frac{3\pi}{2}} + i \sin{\frac{3\pi}{2}}).\)

Step 3: Convert to Standard Form

Converting back to standard form is done by evaluating the real and imaginary parts separately. Here we have \(64(\cos{\frac{3\pi}{2}} + i \sin{\frac{3\pi}{2}}) = 64(0 + i(-1)) = 0 - 64i.\)

Key Concepts

Complex Numbers

Complex numbers are numbers that have both a real and an imaginary part, of the form:

• The real part, often denoted by 'a', represents a number on the standard number line.
• The imaginary part, denoted by 'b', includes the imaginary unit 'i', where 'i' is defined as the square root of -1.

A complex number is typically represented as 'a + bi', where 'a' and 'b' are real numbers. For example, the complex number 2 + 2i:

• Here, the real part 'a' is 2.
• The imaginary part 'b' is also 2.

Combining the real and imaginary parts allows us to work in two dimensions, providing rich possibilities in mathematics and engineering. Understanding complex numbers is crucial, as they lay the foundation for more advanced concepts like polar forms.

Polar Form

The polar form of a complex number provides a different way to express it, based on magnitude and angle:

• Magnitude (or modulus), denoted as 'r', is the distance of the complex number from the origin in a complex plane. It's calculated with: \( r = \sqrt{a^2 + b^2} \).
• Argument (or angle), denoted as '\( \theta \)', is the angle formed with the positive real axis. It is found using: \( \theta = \arctan{(\frac{b}{a})} \).

The polar form represents complex numbers as \( r(\cos{\theta} + i \sin{\theta}) \). This form easily accommodates multiplication and exponentiation, as demonstrated by DeMoivre's Theorem. In our example, the complex number 2 + 2i converts to polar form as \( 2\sqrt{2}(\cos{\frac{\pi}{4}} + i \sin{\frac{\pi}{4}}) \). This transformation simplifies working with powers of the complex number.

Standard Form

In many instances, it's helpful or necessary to convert complex numbers back into their standard form (a + bi):

• This format separates the real and imaginary parts, making certain mathematical manipulations and interpretations easier.

When handling complex numbers, operations such as addition, subtraction, and simplification are straightforward in standard form. After using polar form for operations like multiplication or exponentiation, it often makes sense to return to standard form for clarity or final results.
In our example, after raising the polar form of 2 + 2i to the sixth power using DeMoivre's Theorem, we find that \(64(\cos{\frac{3\pi}{2}} + i \sin{\frac{3\pi}{2}})\) equals \(0 - 64i\). This solution is expressed in standard form as simply -64i, where the real part is 0 and the imaginary part is -64.

Magnitude and Argument

The magnitude and argument are vital components of a complex number when expressed in polar form:

• The magnitude or modulus (r) measures the size or absolute value of the complex number. It's computed with \( r = \sqrt{a^2 + b^2} \), showing how far the number is from the origin.
• The argument (\( \theta \)) tells us the direction of the number from the positive real axis, typically calculated by \( \theta = \arctan{(\frac{b}{a})} \).

These elements provide the geometric interpretation of complex numbers, useful for operations like finding powers using DeMoivre's Theorem. For our original problem 2 + 2i, the magnitude is \(2\sqrt{2}\), and the argument is \(\frac{\pi}{4}\). Combining these in polar form simplifies the manipulation of complex numbers, especially in multiplying or raising them to a power.