We have seen in SAQ 18 of Section 3.4 how some sets of points of the complex plane can be described algebraically in terms of operations on **C**. We now use the modulus function to take this a step further by defining **discs** in the complex plane. As we shall see, discs are extensively used in arguments involving l

3.4 Self-assessment questions and problems

## SAQ 13

Find |*z*| and Arg *z* in each of the following cases.

- Author(s):
In this section we have seen a correspondence between complex numbers and points in the plane using Cartesian coordinates; the real part of the complex number is represented on the real axis (“horizontal”) and the imaginary part on the imaginary axis (“vertical”). We can also use polar coordinates (

*r*,*θ*) in which case*r*, the modulus of a non-zero complex number*z*is positive and*θ*is an argument of*z*, defined only to within an additive intege3.2 Relationship between complex numbers and points in the plane

We have seen in Section 2.2 that the complex number system is obtained by defining arithmetic operations on the set

**R**×**R**. We also know that elements of**R**×**R**can be represented as points in a plane. It seems reasonable to ask what insight can be obtained by representing complex numbers asIn this section we have seen that the complex number system is the set

**R**×**R**together with the operations + and × defined byFrom this, one can justify the performance of ordinary algebraic operations on expressions of the form

You have almost certainly met complex numbers before, but you may well not have had much experience in manipulating them. In this course we provide you with an opportunity to gain confidence in working with complex numbers by working through a number of suitable problems.

Perhaps the most striking difference between real numbers and complex numbers is the fact that complex numbers have a two-dimensional character, arising from our definition of a complex number as

**an ordered pair of****After studying this course, you should be able to:****perform basic algebraic manipulation with complex numbers****understand the geometric interpretation of complex numbers****know methods of finding the nth roots of complex numbers and the solutions of simple polynomial equations.**

**These extracts are from M208 © 2006 The Open University.****All material contained within this course originated at The Open University.****Course image: Matt in Flickr made available under Creative Commons Attribution 2.0 Licence.****Don't miss out:****If reading****Study another free course****There are more than****800 courses on OpenLearn**for you to**This free course provided an introduction to studying Mathematics. It took you through a series of exercises designed to develop your approach to study and learning at a distance and helped to improve your confidence as an independent learner.****Section 6 contains solutions to the exercises that appear throughout sections 1-5.****Click the link below to open the solutions (13 pages, 232KB).****In Section 5 we show how functions may be used to sketch curves in the plane, even when these curves are not necessarily the graphs of functions.****Click the link below to open Section 5 (8 pages, 151KB).****In Section 4 we introduce the***hyperbolic functions*sinh, cosh and tanh, which are constructed from exponential functions. These hyperbolic functions share some of the properties of the trigonometric functions but, as you will see, their graphs are very different.**Click the link below to open Section 4 (5 pages, 104KB).****In Section 1 we formally define***real functions*and describe how they may arise when we try to solve equations. We remind you of some basic real functions and their graphs, and describe how some of the properties of these functions are featured in their graphs.**Click the link below to open Section 1 (12 pages, 1.8MB).****After studying this course, you should be able to:****understand the definition of a real function****use the notation for intervals of the real line****recognise and use the graphs of the basic functions described in the audio section****understand the effect on a graph of translations, scalings, rotations and reflections****understand how the shape of a graph of a function features properties of the function such as increasing, decr**

**Except for third party materials and otherwise stated (see terms and conditions), this content is made available under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 Licence****Study another free course****There are more than****800 courses on OpenLearn**for you t**This free course provided an introduction to studying Mathematics. It took you through a series of exercises designed to develop your approach to study and learning at a distance and helped to improve your confidence as an independent learner.****Activity 24****A new train operator boasts ‘Train times reduced by 12%’. Decrease 90 minutes by 12%. Give your answer as minutes and seconds.**### Answer

3.4 Decreasing by a percentage

Discount can be calculated in the same way as an increase by a percentage. For example, £8 with 15% discount means you actually pay

£8 less (15% of £8)

15% of 8 = × 8 =

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