Q9P - Mathematical Methods in Physical Sciences

Question

Express the following complex numbers in the $\mathit{x}\mathbf{+}\mathit{i}\mathit{y}$ form. Try to visualize each complex number, using sketches as in the examples if necessary. The first twelve problems you should be able to do in your head (and maybe some of the others—try it!) Doing a problem quickly in your head saves time over using a computer. Remember that the point in doing problems like this is to gain skill in manipulating complex expressions, so a good study method is to do the problems by hand and use a computer to check your answers.

 

9. $\mathbf{2}{\mathit{e}}^{\mathbf{-}\mathbf{i}\mathbf{\pi }\mathbf{/}\mathbf{2}}$

Step-by-Step Solution

Answer

The complex number $x+iy$ in form is $z=-2i$ and the graph is shown below:

Step 1: Given information

The given complex number is$2e^{-i\mathrm{\pi }/2}$.

Step 2: Definition of complex numbers

The numbers that are presented in the form of $\mathit{a}\mathbf{+}\mathit{i}\mathit{b}$, where, $\mathit{a}\mathbf{,}\mathit{b}$ are the real numbers and $\mathbf{\text{'}}\mathit{i}\mathbf{\text{'}}$ is an imaginary number called complex numbers.

Step 3: Finding the general form x + i y

Let the given complex number be$\mathit{z}\mathbf{=}\mathbf{2}{\mathit{e}}^{\mathbf{i}\mathbf{\pi }\mathbf{/}\mathbf{2}}$.

Polar form of the complex number is$z=r\left(\cos \left(\theta \right)+i\sin \left(\theta \right)\right)$.

Compare the given complex number with the above two forms.

$$\begin{gathered} r=2 \\ \theta =-\frac{\mathrm{\pi }}{2} \end{gathered}$$

The value of $x,y$ is $r\cos \left(\theta \right),r\sin \left(\theta \right)$ respectively.

$$\begin{gathered} x=r\cos \left(\theta \right) \\ y=r\sin \left(\theta \right) \end{gathered}$$

Put the values of $r$ and$\theta$ in the above equations.

$$\begin{gathered} x=2\cos \left(-\frac{\mathrm{\pi }}{2}\right) \\ x=0 \\ y=2\sin \left(-\frac{\mathrm{\pi }}{2}\right) \\ y=-2 \end{gathered}$$

The general form is $-2i$.

Step 4: Draw the graph of the complex number

The plot of the complex number $-2i$ is shown below by using computer:

Therefore, the general form is$z=-2i$.