Exercises – Long Day

Exercises – Long Day#

Exercise 1: The number \(i\)#

In this exercise you will gain some initial insight into the basics of the complex numbers.

Question a#

What are \(i^2\), \(i^3\), \(i^4\), \(i^5\), \((-i)^2\), \((-i)^3\), \((-i)^4\) and \((-i)^{-5}\)? (Note the negative exponent in the last number.)

Hint

If you are in doubt about how to simplify \(i^2\), then see Definition 4.1.1 in the textbook.

Answer

\[-1,-i,1,i,-1,i,1,i\]

Question b#

What are the real part and the imaginary part of the three complex numbers \(10+i\), \(3\) and \(i\)? What about the real part and imaginary part of the number \(-5-i7\,\)?

Hint

If you are in doubt about what the real part and the imaginary part of a complex number are, then read the bit in the textbook between Definition 4.1.1 and Example 4.1.1.

Remember that the imaginary unit \(i\) is not itself included in the imaginary part (in fact, the imaginary part is real).

Answer

\(10+i\): The real part is \(10\) and the imaginary part \(1\).

\(3\): The real part is \(3\) and the imaginary part \(0\).

\(i\): The real part is \(0\) and the imaginary part \(1\).

\(-5-i7\): The real part is \(-5\) and the imaginary part \(-7\).

Question c#

What are \(\mathrm{Re}(-5-7i)\) and \(\mathrm{Im}(-5-7i)\)?

Answer

\(\mathrm{Re}(-5-7i)=-5\) and \(\mathrm{Im}(-5-7i)=-7\).

Question d#

Write the complex numbers \(7i-5\), \(i(7i-5)\) and \(i(7i-5)i\) in rectangular form.

Hint

Definition 4.2.2 explains how two complex numbers are multiplied.

Answer

\[-5+7i \, \, , \, \, -7-5i \, \, \text{and} \, \, 5-7i.\]

Exercise 2: Brackets and the order of operations#

Question a#

Given the number \(z=3(i-10)-5(7-2i)-i(3i-5)+3i(i-5),\) determine the rectangular form of \(z\).

Hint

In expressions where both multiplication and addition/subtraction are involved, multiplication is to be performed first.

Answer

\[z=-65 +3i.\]

Question b#

We are given the numbers

\[a=5-i(3-i)+6i\,\,\, \mathrm{and} \,\,\,b=-5-4(-2i+1)\,.\]

Write the number \(\,z=a+ib\,\) in rectangular form.

Answer

\[z=-4-6i.\]

Exercise 3: Fractions#

Question a#

Determine the real part and the imaginary part of \((-2+3i)/i\) and write the number in rectangular form.

Hint

You can find some examples of how to rewrite a fraction of complex numbers in rectangular form in Example 4.2.3 in the textbook.

Answer

\[\frac{-2+3i}i=3+2i.\]

Question b#

Reduce the following expression and write it in rectangular form:

\[\frac{3}{5}- \frac{3-2i}{2+i}\,.\]

Hint

It may be easiest to start with the second fraction and rewrite it in rectangular form, using an approach similar to the one in Question a.

Answer

\[\frac{3}{5}- \frac{3-2i}{2+i}=-\frac{1}{5} + \frac{7}{5} i.\]

Exercise 4: The complex plane#

Question a#

Consider the following ten numbers: \(-2,0,i,2-i,1+2i,1,-2+3i,-5i,3\) and \(-1-2i\).

Which of them are complex, which are real and which are purely imaginary?

Draw the ten numbers in the complex plane.

Hint

If you need to brush up on how to draw numbers in the complex plane or on what exactly “purely imaginary” means, have a look in Section 4.1 in the textbook, in particular the text after Definition 4.1.1.

Answer

All ten numbers are complex numbers. The numbers \(-2,0,1,3\) are real numbers. The numbers \(0,i,-5i\) are purely imaginary numbers.

Note that \(0\) can be considered as both a real number and a purely imaginary number since it is located both on the real axis and on the imaginary axis.

Question b#

We are given the number \(z=4+i\,\).

  1. Draw the four numbers \(z,iz,i^2z\) and \(i^3z\) in the complex plane.

  2. What happens geometrically in the complex plane when a complex number is multiplied by \(i\)?

  3. And divided by \(i\)?

Exercise 5: Basic arithmetic#

Question a#

Use elementary calculations to determine the rectangular form of the following complex numbers.

  1. \((5+i)(1+9i).\)

  2. \(i+i^2+i^3+i^4.\)

  3. \(\displaystyle{\frac{1}{1+3i}+\frac{1}{(1+3i)^2}}.\)

  4. \(\displaystyle{\frac{1}{(1+i)^4}}.\)

  5. \(\displaystyle{\frac{5+i}{2-2i}}.\)

  6. \(\displaystyle{\frac{3i}{4}}\,\) and \(\displaystyle{\frac{i2}{4}}.\)

Hint

To simplify a fraction where both numerator and denominator are complex numbers in rectangular form, Equation (4.2) in the textbook will prove helpful. See also Example 4.2.3.

Answer

  1. \(-4+46i\)

  2. \(0\)

  3. \(\frac{1}{50} -\frac{9}{25} i\)

  4. \(- \frac{1}{4}\)

  5. \(1+ \frac{3}{2}i\)

  6. \(\frac{3}{4} i\) and \(\frac{1}{2} i\)

Question b#

We are given two real numbers \(a\) and \(b\,\) that are not both equal to \(0\).

  1. Why is the number \(\,\,\displaystyle{\frac{1}{a+ib}}\,\,\) not in rectangular form?

  2. Compute \(\mathrm{Re}\displaystyle{\left(\frac{1}{a+ib}\right)}\,\) and \(\mathrm{Im}\displaystyle{\left(\frac{1}{a+ib}\right)}\,\,\).

Answer

\(\mathrm{Re}\displaystyle{\left(\frac{1}{a+ib}\right)}\, = \frac{a}{a^2+b^2}\) and \(\mathrm{Im}\displaystyle{\left(\frac{1}{a+ib}\right)} = -\frac{b}{a^2+b^2}\).

Exercise 6: Conjugation#

In Definition 4.2.3 the complex conjugate \(\overline{z}\) of a complex number \(z\) is defined. You might want to read this definition again before continuing so that you have the precise meaning of the notation \(\overline{z}\) fresh in mind.

Question a#

Show that \(\overline{\overline{2+3i}}=2+3i\) and that \(\overline{(2+3i)\cdot (2+3i)}=\overline{2+3i}\cdot \overline{2+3i}\). Now show that in general we have \(\overline{\overline{z}}=z\) and \(\overline{z_1\cdot z_2}=\overline{z_1}\cdot\overline{z_2}\) for all complex numbers \(z\), \(z_1\) and \(z_2\).

Hint

If \(z=a+bi\) is a complex number in rectangular form, what happens if you conjugate twice in a row? In a similar manner, try computing both \(\overline{z_1\cdot z_2}\) and \(\overline{z_1}\cdot\overline{z_2}\) where \(z_1=a+bi\) and \(z_2=c+di\) are the numbers \(z_1\) and \(z_2\) written in rectangular form.

Question b#

Let \(z=a+ib\neq 0\) be a given non-zero complex number. Which complex number is the reflection of \(z\) about

  1. the origin in the complex plane,

  2. the real axis,

  3. the imaginary axis, and

  4. the line through the origin with a slope of \(1\)?

Provide the answers both in rectangular form and as expressions written in terms of \(z\), \(\overline{z}\), and \(i\).

Hint

Start with a clear overview by drawing everything mentioned in the complex plane.

Answer

  1. Rectangular form: \(-a-bi\). Expression: \(-z\).

  2. Rectangular form: \(a-bi\). Expression: \(\overline{z}\).

  3. Rectangular form: \(-a+bi\). Expression: \(-\overline{z}\).

  4. Rectangular form: \(b+ai\). Expression: \(i\cdot \overline{z}\).

Exercise 7: Absolute value#

The absolute value \(\,\left|z\right|\,\) of a complex number \(z\) can be interpreted geometrically as the distance between \(0\) (the origin) and \(z\) in the complex plane. See Figure 4.4 in the textbook for an illustration. The absolute value is also called the modulus in the context of complex numbers.

Question a#

We are given real numbers \(a,b\) and a complex number in rectangular form \(z=a+ib\). Compute \(\left|z\right|\). What is \(|2+3i|\)?

Answer

\(|z|=\sqrt{a^2+b^2}\) and \(|2+3i|=\sqrt{2^2+3^2}=\sqrt{13}\).

Question b#

Investigate the geometric meaning of the absolute value \(\left|z_1-z_2\right|\) for two arbitrary complex numbers \(z_1\) and \(z_2\). Illustrate with examples.

Answer

\(\,\left|z_1-z_2\right|\,\) is the distance between \(z_1\) and \(z_2\) in the complex plane.

Question c#

A set in the complex plane is given by

\[\big\{z \in {\mathbb C} \, \mid \, |z-1|\, = \, 3\big\}\,.\]

Provide a geometric description of the set.

Answer

The set describes a circle in the complex plane centred at the number \(1\) with a radius of \(3\).

Exercise 8: Sets in the complex plane#

In the complex plane we consider the set of numbers \(M=\left\{z\,|\,\,|z-1+2i|\leq 3\,\right\}\).

Question a#

Describe \(M\) and sketch \(M\) in the complex plane.

Question b#

Determine \(M \cap \mathbb{R}\), that is, the set that contains all real numbers in \(M\).

Hint

First, sketch \(M \cap \mathbb{R}\) on your drawing from Question a.

Answer

\[M \cap \mathbb{R}=\left[\,1-\sqrt 5\,,\,1+\sqrt 5\,\right]\,.\]

Exercise 9: An equation involving modulus#

Determine all complex numbers \(z\) that satisfy the equation \(|z-1|=|2z-3|\). Sketch the solution set in the complex plane.

Hint

It may help to write \(z\) in rectangular form, \(z=a+bi\).

Hint

After substituting \(z=a+bi\), check that the equation \(|z-1|=|2z-3|\) holds if and only if \((a-1)^2+b^2=(2a-3)^2+(2b)^2\).

Answer

It can be shown that \((a-1)^2+b^2=(2a-3)^2+(2b)^2\) if and only if \((a-5/3)^2+b^2=(1/3)^2\). Hence the solution set forms a circle in the complex plane with a radius of \(1/3\) centred at the real number \(5/3\).

Have you finished all the exercises?#

If it went too quickly and you are eager for more, then click here for some challenging optional extra exercises.

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