Calculating the Fourth Root

Oct 13, 2025 by wbrum.org 17 views

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Introduction

De Moivre's theorem is a fundamental concept in mathematics, particularly in the field of complex numbers. It states that for any complex number $z = r(\cos \theta + i\sin \theta)$ , the $n$ th power of $z$ can be expressed as $z^n = r^n(\cos n\theta + i\sin n\theta)$ . In this article, we will use De Moivre's theorem to find the fourth root of $-i$ .

Background

To begin, let's recall the definition of a complex number. A complex number is a number that can be expressed in the form $a + bi$ , where $a$ and $b$ are real numbers and $i$ is the imaginary unit, which satisfies $i^2 = -1$ . The complex number $-i$ can be expressed as $0 - i$ , where $0$ is the real part and $-1$ is the imaginary part.

Expressing $-i$ in Polar Form

To apply De Moivre's theorem, we need to express $-i$ in polar form. The polar form of a complex number is given by $r(\cos \theta + i\sin \theta)$ , where $r$ is the magnitude of the complex number and $\theta$ is the argument. In this case, the magnitude of $-i$ is $1$ , and the argument is $-\frac{\pi}{2}$ .

Applying De Moivre's Theorem

De Moivre's theorem states that for any complex number $z = r(\cos \theta + i\sin \theta)$ , the $n$ th power of $z$ can be expressed as $z^n = r^n(\cos n\theta + i\sin n\theta)$ . To find the fourth root of $-i$ , we need to find the value of $z$ such that $z^4 = -i$ . Using De Moivre's theorem, we can write:

z^4 = 1(\cos (-\frac{\pi}{2}) + i\sin (-\frac{\pi}{2}))

Finding the Fourth Root

To find the fourth root of $-i$ , we need to find the value of $z$ such that $z^4 = -i$ . We can do this by using the formula for the $n$ th root of a complex number:

z = r^{\frac{1}{n}}(\cos \frac{\theta + 2k\pi}{n} + i\sin \frac{\theta + 2k\pi}{n})

where $k$ is an integer. In this case, $r = 1$ , $\theta = -\frac{\pi}{2}$ , and $n = 4$ .

Calculating the Fourth Root

Using the formula for the $n$ th root of a complex number, we can calculate the fourth root of $-i$ :

z = 1^{\frac{1}{4}}(\cos \frac{-\frac{\pi}{2} + 2k\pi}{4} + i\sin \frac{-\frac{\pi}{2} + 2k\pi}{4})

Simplifying the Expression

We can simplify the expression for the fourth root of $-i$ by evaluating the trigonometric functions:

z = \cos \frac{-\frac{\pi}{2} + 2k\pi}{4} + i\sin \frac{-\frac{\pi}{2} + 2k\pi}{4}

Finding the Possible Values

To find the possible values of the fourth root of $-i$ , we need to evaluate the expression for different values of $k$ . We can do this by plugging in different values of $k$ into the expression:

z = \cos \frac{-\frac{\pi}{2} + 2k\pi}{4} + i\sin \frac{-\frac{\pi}{2} + 2k\pi}{4}

Evaluating the Expression for Different Values of $k$

We can evaluate the expression for different values of $k$ to find the possible values of the fourth root of $-i$ :

For $k = 0$ , we have:

z = \cos \frac{-\frac{\pi}{2}}{4} + i\sin \frac{-\frac{\pi}{2}}{4} = \cos (-\frac{\pi}{8}) + i\sin (-\frac{\pi}{8})

For $k = 1$ , we have:

z = \cos \frac{-\frac{\pi}{2} + 2\pi}{4} + i\sin \frac{-\frac{\pi}{2} + 2\pi}{4} = \cos (\frac{3\pi}{8}) + i\sin (\frac{3\pi}{8})

For $k = 2$ , we have:

z = \cos \frac{-\frac{\pi}{2} + 4\pi}{4} + i\sin \frac{-\frac{\pi}{2} + 4\pi}{4} = \cos (\frac{7\pi}{8}) + i\sin (\frac{7\pi}{8})

For $k = 3$ , we have:

z = \cos \frac{-\frac{\pi}{2} + 6\pi}{4} + i\sin \frac{-\frac{\pi}{2} + 6\pi}{4} = \cos (\frac{15\pi}{8}) + i\sin (\frac{15\pi}{8})

Conclusion

In this article, we used De Moivre's theorem to find the fourth root of $-i$ . We expressed $-i$ in polar form and applied De Moivre's theorem to find the value of $z$ such that $z^4 = -i$ . We then evaluated the expression for different values of $k$ to find the possible values of the fourth root of $-i$ . The possible values of the fourth root of $-i$ are:

z = \cos \frac{-\frac{\pi}{2} + 2k\pi}{4} + i\sin \frac{-\frac{\pi}{2} + 2k\pi}{4}

where $k$ is an integer.

References

De Moivre, A. (1730). "Miscellanea Analytica". London: Woodfall.
Euler, L. (1748). "Introductio in Analysin Infinitorum". Lausanne: Marc-Michel Bousquet.
Courant, R., & Robbins, H. (1941). "What is Mathematics?". Oxford University Press.

Note: The references provided are a selection of the many sources that have contributed to the development of De Moivre's theorem and its applications.

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Introduction

In our previous article, we used De Moivre's theorem to find the fourth root of $-i$ . In this article, we will answer some common questions related to this topic.

Q: What is De Moivre's theorem?

A: De Moivre's theorem is a fundamental concept in mathematics, particularly in the field of complex numbers. It states that for any complex number $z = r(\cos \theta + i\sin \theta)$ , the $n$ th power of $z$ can be expressed as $z^n = r^n(\cos n\theta + i\sin n\theta)$ .

Q: How do I express a complex number in polar form?

A: To express a complex number in polar form, you need to find the magnitude and argument of the complex number. The magnitude is the distance from the origin to the point in the complex plane, and the argument is the angle between the positive real axis and the line segment connecting the origin to the point.

Q: What is the polar form of $-i$ ?

A: The polar form of $-i$ is $1(\cos (-\frac{\pi}{2}) + i\sin (-\frac{\pi}{2}))$ .

Q: How do I apply De Moivre's theorem to find the fourth root of $-i$ ?

A: To apply De Moivre's theorem, you need to find the value of $z$ such that $z^4 = -i$ . You can do this by using the formula for the $n$ th root of a complex number:

z = r^{\frac{1}{n}}(\cos \frac{\theta + 2k\pi}{n} + i\sin \frac{\theta + 2k\pi}{n})

where $k$ is an integer.

Q: What are the possible values of the fourth root of $-i$ ?

A: The possible values of the fourth root of $-i$ are:

z = \cos \frac{-\frac{\pi}{2} + 2k\pi}{4} + i\sin \frac{-\frac{\pi}{2} + 2k\pi}{4}

where $k$ is an integer.

Q: How do I evaluate the expression for different values of $k$ ?

A: To evaluate the expression for different values of $k$ , you need to plug in different values of $k$ into the expression:

z = \cos \frac{-\frac{\pi}{2} + 2k\pi}{4} + i\sin \frac{-\frac{\pi}{2} + 2k\pi}{4}

Q: What are the possible values of the fourth root of $-i$ for different values of $k$ ?

A: The possible values of the fourth root of $-i$ for different values of $k$ are:

For $k = 0$ , we have:

z = \cos (-\frac{\pi}{8}) + i\sin (-\frac{\pi}{8})

For $k = 1$ , we have:

z = \cos (\frac{3\pi}{8}) + i\sin (\frac{3\pi}{8})

For $k = 2$ , we have:

z = \cos (\frac{7\pi}{8}) + i\sin (\frac{7\pi}{8})

For $k = 3$ , we have:

z = \cos (\frac{15\pi}{8}) + i\sin (\frac{15\pi}{8})

Q: What is the significance of De Moivre's theorem in mathematics?

A: De Moivre's theorem is a fundamental concept in mathematics, particularly in the field of complex numbers. It has numerous applications in mathematics, physics, and engineering, including the study of periodic phenomena, the solution of equations, and the representation of complex numbers in polar form.

Q: How do I use De Moivre's theorem to solve problems in mathematics?

A: To use De Moivre's theorem to solve problems in mathematics, you need to apply the formula for the $n$ th power of a complex number:

z^n = r^n(\cos n\theta + i\sin n\theta)

You can then use this formula to solve problems involving complex numbers, such as finding the fourth root of $-i$ .

Conclusion

In this article, we answered some common questions related to finding the fourth root of $-i$ using De Moivre's theorem. We provided explanations and examples to help you understand the concept and apply it to solve problems in mathematics.

References

De Moivre, A. (1730). "Miscellanea Analytica". London: Woodfall.
Euler, L. (1748). "Introductio in Analysin Infinitorum". Lausanne: Marc-Michel Bousquet.
Courant, R., & Robbins, H. (1941). "What is Mathematics?". Oxford University Press.

Note: The references provided are a selection of the many sources that have contributed to the development of De Moivre's theorem and its applications.