Find The Indicated Power Using De Moivre's Theorem

Demoiivre's Theorem: Unleashing the Power of Complex Numbers

Demoiivre's Theorem is a powerful tool in the realm of complex numbers, providing a straightforward method for calculating powers of complex numbers expressed in polar form. Understanding and applying this theorem is crucial for anyone studying complex analysis, trigonometry, and various branches of engineering and physics. This comprehensive guide will delve into the theorem itself, its proof, applications, and common pitfalls to ensure a thorough understanding. We'll explore how to find the indicated power of a complex number using De Moivre's Theorem, tackling both simple and more complex examples.

Understanding Complex Numbers and Polar Form

Before diving into De Moivre's Theorem, let's refresh our understanding of complex numbers. A complex number, z, is typically represented as z = a + bi, where a and b are real numbers, and i is the imaginary unit (√-1). The term a is the real part (Re(z)) and b is the imaginary part (Im(z)).

Complex numbers can also be represented in polar form, which offers significant advantages when dealing with powers and roots. In polar form, a complex number is expressed as:

z = r(cos θ + i sin θ)

where:

• r is the modulus (or magnitude) of z, given by r = √(a² + b²). This represents the distance from the origin to the point representing z in the complex plane.
• θ is the argument (or phase) of z, given by θ = arctan(b/a). This represents the angle between the positive real axis and the line connecting the origin to the point representing z in the complex plane. The value of θ needs to be carefully considered to place the point in the correct quadrant.

De Moivre's Theorem: The Statement and its Proof

De Moivre's Theorem states that for any complex number z = r(cos θ + i sin θ) and any integer n, the following equation holds true:

zⁿ = rⁿ(cos(nθ) + i sin(nθ))

This means that raising a complex number to a power n involves raising its modulus to the power n and multiplying its argument by n.

Proof (for positive integer n):

The proof can be elegantly demonstrated using mathematical induction.

• Base Case (n=1): The theorem clearly holds true for n=1, as z¹ = r¹(cos(1θ) + i sin(1θ)) = z.

• Inductive Hypothesis: Assume the theorem holds true for some positive integer k: zᵏ = rᵏ(cos(kθ) + i sin(kθ)).

• Inductive Step: We need to show that the theorem also holds true for n = k + 1:

zᵏ⁺¹ = zᵏ * z = [rᵏ(cos(kθ) + i sin(kθ))] * [r(cos θ + i sin θ)]

Expanding this expression using distributive property and trigonometric identities (specifically, the angle sum identities for cosine and sine):

zᵏ⁺¹ = rᵏ⁺¹ [(cos(kθ)cosθ - sin(kθ)sinθ) + i(cos(kθ)sinθ + sin(kθ)cosθ)]

Using the angle sum identities: cos(A+B) = cosAcosB - sinAsinB and sin(A+B) = sinAcosB + cosAsinB, we get:

zᵏ⁺¹ = rᵏ⁺¹ [cos(kθ + θ) + i sin(kθ + θ)] = rᵏ⁺¹ [cos((k+1)θ) + i sin((k+1)θ)]

This proves the theorem for positive integers n by mathematical induction. The proof for negative integer values of n involves using the definition of negative exponents and the conjugate of the complex number. The extension to rational exponents requires understanding roots of complex numbers and is discussed later.

Applying De Moivre's Theorem: Step-by-Step Guide

Let's illustrate how to apply De Moivre's Theorem with a step-by-step example:

Example: Find (1 + i)⁵ using De Moivre's Theorem.

Step 1: Convert to Polar Form:

First, convert the complex number (1 + i) into polar form.

r = √(1² + 1²) = √2

θ = arctan(1/1) = π/4 (Since 1+i lies in the first quadrant)

Therefore, 1 + i = √2(cos(π/4) + i sin(π/4)).

Step 2: Apply De Moivre's Theorem:

Now, apply De Moivre's Theorem:

(1 + i)⁵ = (√2)⁵ [cos(5 * π/4) + i sin(5 * π/4)]

Step 3: Simplify:

Simplify the expression:

(√2)⁵ = 4√2

cos(5π/4) = -√2/2

sin(5π/4) = -√2/2

Therefore, (1 + i)⁵ = 4√2(-√2/2 - i√2/2) = -4 - 4i

Therefore, (1+i)⁵ = -4 - 4i

Working with Negative and Rational Exponents

While the initial proof focuses on positive integers, De Moivre's Theorem can be extended to negative integers and rational numbers (fractional exponents), although with some nuances.

Negative Exponents: For negative integer exponents, n, we utilize the property that z⁻ⁿ = 1/zⁿ. We apply De Moivre's theorem to find zⁿ and then take the reciprocal.

Rational Exponents (Roots): When dealing with rational exponents, p/q (where p and q are integers and q ≠ 0), we are essentially finding the qth roots of the complex number raised to the power of p. This introduces the concept of multiple roots. A complex number will generally have q distinct qth roots. Each root will have the same modulus, but their arguments will differ by multiples of 2π/q.

Example with a Rational Exponent: Find the cube roots of 8.

8 can be represented in polar form as 8(cos(0) + i sin(0)).

Using De Moivre's Theorem to find the cube roots (n = 1/3):

z^(1/3) = 8^(1/3) [cos(0 + 2kπ)/3 + i sin(0 + 2kπ)/3], where k = 0, 1, 2.

For k = 0: z₁ = 2(cos(0) + i sin(0)) = 2

For k = 1: z₂ = 2(cos(2π/3) + i sin(2π/3)) = -1 + i√3

For k = 2: z₃ = 2(cos(4π/3) + i sin(4π/3)) = -1 - i√3

These are the three cube roots of 8.

Applications of De Moivre's Theorem

De Moivre's Theorem has widespread applications across various fields:

• Trigonometry: It provides a way to derive trigonometric identities for multiple angles. For example, expanding (cos θ + i sin θ)ⁿ and equating the real and imaginary parts yields formulas for cos(nθ) and sin(nθ).

• Complex Analysis: It’s fundamental to understanding complex functions, power series, and contour integration.

• Engineering and Physics: It's crucial in areas like signal processing, electrical engineering (AC circuits), and quantum mechanics, where complex numbers are frequently used to represent waves and oscillations.

Frequently Asked Questions (FAQ)

• Q: Can De Moivre's Theorem be used for non-integer exponents? A: While the initial proof focuses on integer exponents, the theorem can be extended to rational exponents (fractional powers) to find roots of complex numbers, leading to multiple roots. However, it doesn't directly apply to irrational exponents.

• Q: What if the argument θ is negative? A: De Moivre's Theorem works perfectly well with negative arguments. Just substitute the negative angle into the formula.

• Q: How do I choose the correct argument (θ)? A: The argument should be chosen carefully to accurately reflect the location of the complex number in the complex plane. Use the arctangent function (arctan) to find the principal argument. Remember to adjust the argument if necessary to place the point in the correct quadrant.

• Q: What are the limitations of De Moivre's Theorem? A: The theorem primarily applies to finding powers and roots of complex numbers. It doesn't directly solve all problems involving complex numbers, and it becomes more complex when dealing with irrational exponents.

Conclusion

De Moivre's Theorem is a cornerstone of complex number manipulation. Its elegance and efficiency in calculating powers and roots of complex numbers make it an invaluable tool in various fields of mathematics, engineering, and science. By understanding the theorem, its proof, and the nuances of applying it to different types of exponents, you'll equip yourself with a powerful technique to tackle challenging problems involving complex numbers. Remember the step-by-step approach: convert to polar form, apply the theorem, and simplify the resulting expression. Mastering this theorem unlocks a deeper understanding of the rich world of complex numbers.