Problem 36 - Advanced Engineering Mathematics

Question

Express the given complex number first in polar form and then in the form \(a+i b\). $$ \frac{\left[8\left(\cos \frac{3 \pi}{8}+i \sin \frac{3 \pi}{8}\right)\right]^{3}}{\left[2\left(\cos \frac{\pi}{16}+i \sin \frac{\pi}{16}\right)\right]^{10}} $$

Step-by-Step Solution

Answer

The final expression in rectangular form is \( \frac{i}{2} \).

Step 1: Identify Polar Forms

The given complex numbers are already in polar form:- \( z_1 = 8 \left(\cos \frac{3\pi}{8} + i \sin \frac{3\pi}{8} \right) \) with modulus \( r_1 = 8 \) and argument \( \theta_1 = \frac{3\pi}{8} \).- \( z_2 = 2 \left(\cos \frac{\pi}{16} + i \sin \frac{\pi}{16} \right) \) with modulus \( r_2 = 2 \) and argument \( \theta_2 = \frac{\pi}{16} \).

Step 2: Apply De Moivre's Theorem

Calculate each power using De Moivre's Theorem:- \( z_1^3 = 8^3 \left(\cos \left( 3 \times \frac{3\pi}{8} \right) + i \sin \left( 3 \times \frac{3\pi}{8} \right) \right) \).- \( z_2^{10} = 2^{10} \left(\cos \left( 10 \times \frac{\pi}{16} \right) + i \sin \left( 10 \times \frac{\pi}{16} \right) \right) \).

Step 3: Simplify the Powers

Calculate each power:- \( 8^3 = 512 \).- \( 2^{10} = 1024 \).Calculate the arguments:- The argument for \( z_1^3 \) is \( \frac{9\pi}{8} \).- The argument for \( z_2^{10} \) is \( \frac{10\pi}{16} = \frac{5\pi}{8} \).

Step 4: Divide the Polar Forms

Divide the moduli and subtract the arguments for the quotient:\[ \frac{z_1^3}{z_2^{10}} = \frac{512}{1024} \left( \cos \left( \frac{9\pi}{8} - \frac{5\pi}{8} \right) + i \sin \left( \frac{9\pi}{8} - \frac{5\pi}{8} \right) \right) = \frac{1}{2} \left( \cos \frac{4\pi}{8} + i \sin \frac{4\pi}{8} \right) \].Simplify the argument: \( \cos \frac{\pi}{2} = 0 \) and \( \sin \frac{\pi}{2} = 1 \).

Step 5: Convert to Rectangular Form

Convert the polar form to rectangular form:\[ \frac{1}{2} \left( 0 + i \cdot 1 \right) = \frac{i}{2} \].

Key Concepts

Polar Form

The polar form of complex numbers expresses them in terms of a magnitude and angle. It is particularly useful when dealing with multiplication and division of complex numbers, as well as calculating powers and roots. In the polar form, a complex number is represented as

• \( z = r \left(\cos \theta + i \sin \theta\right) \)

where \( r \) is the modulus (absolute value) of the complex number, and \( \theta \) is the argument (angle in radians). The modulus \( r \) can be found using the formula \(\sqrt{a^2 + b^2}\), while the argument \( \theta \) is \(\tan^{-1} \left( \frac{b}{a} \right)\) in the standard Cartesian (rectangular) form \( a + ib \). Adding compactness and clarity, polar coordinates are pivotal in simplifying operations on complex numbers, especially when raising them to a power or finding roots.

De Moivre's Theorem

De Moivre's Theorem beautifully links complex numbers in polar form to the world of trigonometry and exponents. It states that for a complex number \( z = r(\cos \theta + i \sin \theta) \) and any integer \( n \), the power of \( z^n \) is computed as

• \[ z^n = r^n \left( \cos(n\theta) + i \sin(n\theta) \right) \]

This theorem makes it remarkably straightforward to compute powers of complex numbers. Simply raise the modulus to the desired power \( r^n \) and multiply the argument \( \theta \) by the same power \( n \). De Moivre's Theorem also allows for efficient division and multiplication in polar coordinates by manipulating the modulus and argument directly. It's an elegant way of handling complex powers without delving into more cumbersome rectangular computations.

Rectangular Form

The rectangular form of a complex number, also known as Cartesian form, is simply the expression

• \( a + ib \)

where \( a \) represents the real component, and \( b \) represents the imaginary component. This is the standard form most people first encounter when learning about complex numbers. Converting from polar to rectangular involves calculating the real and imaginary parts using

• \( a = r \cos \theta \)
• \( b = r \sin \theta \)

One of the powerful aspects of rectangular form is its intuitive resemblance to coordinates in a plane, making it easy to visualize and interpret in geometric terms. However, for operations like multiplication, division, and exponentiation, the polar representation often proves more efficient.

Complex Arithmetic

Arithmetic involving complex numbers, whether in polar or rectangular form, follows specific rules. In rectangular form:

• Addition/Subtraction: \(a + ib\) and \(c + id\) result in \((a+c) + i(b+d)\).
• Multiplication: \(a + ib\)(c + id) = \(ac-bd) + i(ad+bc)\).
• Division requires multiplying by the complex conjugate.

In polar form, operations become even more streamlined:

• Multiplication: Simply multiply the moduli and add the arguments.
• Division: Divide the moduli and subtract the arguments.

This representation is often more elegant, especially for more complex operations such as taking roots or raising numbers to a power, as demonstrated by De Moivre's Theorem. Both forms have their unique benefits and can be chosen based on the complexity and nature of the calculations at hand.