Complex numbers

Definition

Complex numbers are numbers that combine a real part and an imaginary part, expressed in the form a + bi, where 'a' is the real component and 'bi' is the imaginary component (with 'i' being the imaginary unit, defined as the square root of -1). They extend the concept of one-dimensional number lines to a two-dimensional plane, allowing for operations that involve angles and magnitudes, closely connecting them to trigonometry and polar coordinates.

5 Must Know Facts For Your Next Test

1. Complex numbers can be added, subtracted, multiplied, and divided using specific rules that incorporate both real and imaginary parts.
2. The magnitude (or modulus) of a complex number a + bi is calculated using the formula $$|z| = \sqrt{a^2 + b^2}$$.
3. In polar coordinates, a complex number's position can be described using its magnitude and angle relative to the positive x-axis, which provides insights into its behavior under multiplication and rotation.
4. Euler's formula connects complex numbers with trigonometry: $$e^{i\theta} = \cos(\theta) + i\sin(\theta)$$, demonstrating how exponential functions relate to rotations in the complex plane.
5. The conjugate of a complex number a + bi is given by a - bi, which is useful in division and simplifying expressions involving complex numbers.

Review Questions

• How do you add and subtract complex numbers, and what geometric interpretation can you provide?
  • To add complex numbers, simply combine their real parts and their imaginary parts separately. For example, (a + bi) + (c + di) results in (a + c) + (b + d)i. Geometrically, this can be visualized as placing vectors in the Argand plane; the resulting vector from the origin to the point representing the sum is found by connecting the tips of the individual vectors. Subtraction follows the same process but involves subtracting respective components.
• Explain how to convert a complex number from rectangular form to polar form and why this is useful.
  • To convert a complex number from rectangular form (a + bi) to polar form, calculate its magnitude $$r = \sqrt{a^2 + b^2}$$ and angle $$\theta = \tan^{-1}(\frac{b}{a})$$. The polar form is then expressed as $$r(cos \theta + i sin \theta)$$ or $$r e^{i\theta}$$. This conversion is useful because it simplifies multiplication and division of complex numbers, turning them into problems of adding angles and multiplying magnitudes.
• Analyze how Euler's formula links complex numbers with trigonometric functions and its implications for rotations in the Argand plane.
  • Euler's formula states that $$e^{i\theta} = \cos(\theta) + i\sin(\theta)$$, illustrating a deep connection between exponential functions, trigonometry, and complex numbers. This relationship implies that multiplying a complex number by another in polar form involves adding their angles while multiplying their magnitudes. Consequently, this provides a method for visualizing rotations in the Argand plane; rotating by an angle $$\theta$$ corresponds directly to moving around a circle with radius equal to the magnitude of the complex number.

Related terms

Imaginary unit: The imaginary unit 'i' is defined as the square root of -1, which allows for the existence of imaginary numbers and is essential in forming complex numbers.

Polar form: The polar form of a complex number expresses it in terms of its magnitude and angle, often represented as r(cos θ + i sin θ) or simply r e^(iθ), linking complex numbers to trigonometric functions.

Argand plane: The Argand plane is a graphical representation of complex numbers, where the x-axis represents the real part and the y-axis represents the imaginary part, facilitating operations like addition and multiplication geometrically.