# Mathematics for Technology II (Math 2131)

Durham College, Mathematics
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• 14 week duration
• ##### Solving Systems of Equations

This unit introduces how to systematically solve a system of equations, namely linear equations. Examples of non-linear systems, including systems of 3 unknowns will be of emphasis.

• ##### Graphs of Trigonometric Functions

The unit focuses primarily on how to graph periodic sinusoidal functions, and how to identify features of a waveform to produce an equation by inspection.

• ##### Polar Coordinate Functions

An introduction to the polar coordinate system.

• ##### Complex Numbers

This unit is an extension of what was introduced in Math 1131. To learn how to work with radicals, knowing your exponent laws in crucial. Hence, this unit begins with a thorough review.

• ##### Logarithmic Functions

This chapter introduces you to exponential functions, and how they can be solved using logarithms.

• ##### Trigonometric Identities and Equations
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• ##### Analytic Geometry
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## Mathematics for Technology II (Math 2131)

### Introduction to Complex Numbers

Up to this point, we have avoided taking the square root of negative numbers, such as √(−1). We’ve simply dismissed the notion by saying an output does not exist, and rightfully so given the real number system we’ve been brought up with. In this section, we deal with them by introducing a new number system known as imaginary numbers.

# Out of curiosity, can you find a real number, when multiplied in itself gets you  “-4”?

… probably not because there are no real numbers that can get you that.

Here’s a historical link as to why we need a new number system.

When math was first invented, early mathematicians tried solving equations like x − 5 = 0, so they asked, what is x? The naturals solved them (easily).

• Then we asked, “what about x + 5 = 0?” So, negative numbers were invented.
• Then we asked “what about 2x = 1?” So, rational numbers were invented.
• Then we asked “what about x² = 2?” So, irrational  numbers were invented.

This is the only question that was left, so mathematicians decided to invent the “imaginary” numbers to solve it. All the other numbers, at some point, didn’t exist and didn’t seem “real”, but now they’re fine.

Now that we have imaginary numbers, we can ‘solve’ every polynomial, so it makes sense that that’s the last place to stop.

A second interpretation

Another way to look at it is to ask yourself, do negative numbers really exist.

• Of course they do not. You can’t have a negative number of apples.

Yet, the beauty of negative numbers is that when we define them, then all of a sudden we can use them to solve problems we were never ever able to solve before, or we can solve them in a much simpler way. Negative numbers were invented as a tool to help us solve real life problems.

Therefore, the idea that imaginary numbers exist or not has no bearing on whether they are actually useful in solving the problems of every day life, or making them many times more easy to solve.

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Definition of Imaginary Numbers

Recall that in the real number system, the equation

${x}^{2}=–1\phantom{\rule{0ex}{0ex}}$

Has no solution because there was no real number such that its square is –1.

Now, if you run into this situation, we can use a letter i to denote the answer  to this. Therefore, we call i an imaginary unit that represents:

$i=\sqrt{–1}\phantom{\rule{0ex}{0ex}}$

Definition of Complex Numbers

A complex number is any number, real and/or imaginary, that can be written in the form:

$\mathbit{a}+\mathbit{b}i$
• where a and b are real numbers, and i = √(–1) is the imaginary unit. A complex number written this way is said to be in rectangular form. You’ll find out later how to convert complex numbers written in rectangular form into polar form just like we did with polar equations.

Some examples of complex numbers are shown below. Notice that if a real number exists, it’s written as the first term, followed by the imaginary unit.

• 4+2i
• −7+8i
• 5.92−2.93i
• 27i
• 83

# Combining Complex Numbers

Part of this unit includes knowing how to combine complex numbers via addition and subtraction. Luckily, they’re treated no different than how you combine like-terms in algebra. The following examples are combined in the video below:

 (a) 3i + 5i (b) 2i + (6 – 5i) (c) (2 – 5i) + (–4 + 3i) (d) (–6 + 2i) – (4 – i)

# Raising i to a Power

To understand how to calculate i when raised to any power (i.e. iⁿ), you need to know what happens specifically to i2, i3, and i4. Knowing these will help you understand how to reduce i raised to larger powers. The video below thorough explains how.

Summary:   If we represent n as the exponent of iⁿ,

• When n is 1i = √(–1)
• When n is 2, i² = –1
• When n is 3, i³ = –√(–1) or simply –i
• When n is 4, i4n = 1 (where n > 1, and n is a whole number)

# Graphing Complex Numbers

To graph a complex number, you use axes similar to the rectangular coordinate system. The only difference is that the horizontal axis is the real axis, and the vertical axis is the imaginary axis. Such a coordinate system defines what is called the complex plane. Real numbers are graphed as points on the horizontal axis; pure imaginary numbers are graphed as points on the vertical axis.

To plot a complex number a + bi in the complex plane, simply locate a point with a horizontal coordinate of a and a vertical coordinate of b.

In case you’re asked to graph the conjugate of a complex number in the form a + bi, the conjugate of a complex number is obtained by changing the sign of the imaginary term (↓).