Bestimmen Sie den Betrag der folgenden komplexen Zahlen: $$ \begin{array}{lll} z_{1}=4-3 \mathrm{j} . & z_{2}=-2-6 \mathrm{j}, & z_{3}=3\left(\cos 60^{\circ}-\mathrm{j} \cdot \sin 60^{\circ}\right) \\ z_{4}=-3+4 \mathrm{j}, & z_{5}=-4 \mathrm{j}, & z_{6}=-3 \cdot \mathrm{e}^{j} 30^{\circ} \end{array} $$

The magnitude, or modulus, of a complex number is a measure of its distance from the origin on the complex plane. You can think of a complex number as a point in a two-dimensional space, with the real part representing the x-coordinate and the imaginary part representing the y-coordinate. The formula to find the magnitude of a complex number in Cartesian form (where the number is written as \(z = a + bj\)) is: \[|z| = \sqrt{a^2 + b^2}\] This formula is rooted in the Pythagorean theorem. It allows us to calculate the distance from the point (a, b) to the origin (0, 0). For example, if we have the complex number \(4 - 3j\), we substitute \(a = 4\) and \(b = -3\) into the formula: \[|4 - 3j| = \sqrt{4^2 + (-3)^2} = \sqrt{16 + 9} = \sqrt{25} = 5\] Thus, the magnitude of \(4 - 3j\) is 5.

The Cartesian form of a complex number expresses it in terms of its real part and its imaginary part. This is the form most people start with when learning about complex numbers. It looks like this: \(z = a + bj\), where \(a\) is the real part and \(b\) is the imaginary part (multiplied by \(j\), the imaginary unit). Understanding how to manipulate and interpret complex numbers in Cartesian form is fundamental. For example, with \(z_1 = 4 - 3j\), \(a\) is 4 and \(b\) is -3. The Cartesian form is straight-forward and is useful for basic operations like addition, subtraction, and finding the magnitude.

The polar form of a complex number represents it in terms of its magnitude and angle (or phase). It is especially useful for multiplication and division of complex numbers. The polar form is written as: \[z = r \left( \cos(\theta) + j \sin(\theta) \right)\] Here, \(r\) is the magnitude and \(\theta\) is the angle with the positive x-axis, called the argument. For example, \(z_3 = 3 \left( \cos 60^{\circ} - j \sin 60^{\circ} \right)\) is already in polar form. We know \(\cos 60^{\circ} = \frac{1}{2}\) and \(\sin 60^{\circ} = \frac{\sqrt{3}}{2}\), so converting it back to Cartesian form would give us \(\frac{3}{2} - j\frac{3\sqrt{3}}{2}\). Converting between Cartesian and polar forms provides deeper insights and different tools for solving complex number problems.

The exponential form of a complex number is another way to represent complex numbers, especially when dealing with problems involving powers and roots. This form leverages Euler's formula: \[e^{j\theta} = \cos \theta + j \sin \theta\] In this form, a complex number is written as: \[z = re^{j\theta}\] where \(r\) is the magnitude and \(\theta\) is the argument. For example, consider \(z_6 = -3 \cdot e^{j 30^{\circ}}\). The magnitude of \(z_6\) is simply the coefficient, which is 3, as the exponential part represents rotation on the complex plane. The exponential form is efficient for multiplication and division of complex numbers and for raising them to powers.