1. Lesson 6 – Complex Numbers
Further Pure 1
Lesson 6 – Complex Numbers

2. Modulus/Argument of a complex number
The position of a complex number (z) can be represented by the distance that z is from the origin (r) and the angle made with the positive real axis (θ).
Distance is given by r = |z|
r = |z| = √(x2+y2)
r is known as the modulus of a complex number. Im r y θ x Re

3. Argument of a complex numberIm
Now lets look at the angle (θ) that the line makes with the positive real axis.
NOTE: θ is measured in radians in an anticlockwise direction from the positive real axis.
θ is measured from –π to π and is known as the principal argument of z.
Argument z = arg z = θ
y = r sinθ, x = r cosθ
tan θ = y/x
θ = inv tan (y/x) r y θ x Re

4. Modulus/Argument of a complex number
Calculate the modulus and argument of the following complex numbers. (Hint, it helps to draw a diagram)
1) 3 + 4j |z| = √(32+42) = 5
arg z = inv tan (4/3)
= 0.927
2) 5 - 5j |z| = √(52+52) = 5√2
arg z = inv tan (5/-5)
= -π/4
3) -2√3 + 2j |z| = √((2√3)2+22) = 4
arg z = inv tan (2/-2√3)
= 5π/6

5. Modulus/Argument of a complex number
Note, inv tan (y/x) will return answer in first/fourth quadrant.
Last example on previous slide inv tan (2/-2√3) on your calculator will return, -π/6, however the answer we want is 5π/6.
In some circumstances you way need to add or subtract π.
This is why a diagram is useful.

6. Modulus-Argument form of a complex number
So far we have plotted the position of a complex number on the Argand diagram by going horizontally on the real axis and vertically on the imaginary.
(This is just like plotting co-ordinates on an x,y axis)
However it is also possible to locate the position of a complex number by the distance travelled from the origin (pole), and the angle turned through from the positive x-axis.
(This is sometimes known as Polar co-ordinates and can be studied in another course)

7. Modulus-Argument form of a complex number
(x,y)
(r, θ)
cosθ = x/r, sinθ = y/r
x = r cosθ, y = r sinθ, y Im Im θ y x r y θ x Re Re

8. Converting Converting from Cartesian to Polar
(x,y) = [√(x2+y2),(inv tan (y/x))] = (r, θ)
Convert the following from Cartesian to Polar
i) (1,1) = (√2,π/4)
ii) (-√3,1) = (2,5π/6)
iii) (-4,-4√3) = (8,-2π/3) Im r y θ x Re

9. Converting (r, θ) = (r cosθ, r sin θ)
Converting from Polar to Cartesian
(r, θ) = (r cosθ, r sin θ)
Convert the following from Polar to Cartesian
i) (4,π/3) = (2,2√3)
ii) (3√2,-π/4) = (3,-3)
iii) (6√2,3π/4) = (-6,6) Im r y θ x Re

10. Using the Calculator
It is possible to convert from Cartesian to Polar and back using your calculator.
(Casio fx-83MS – other calculators may be different)
Make sure that your calculator is in Radians.
The polar coordinates (√2,π/4) equal the Cartesian co-ordinates (1,1)
On the calculator press
The answer shown is the first Cartesian co-ordinate x. To view the y press RCL F. To view the x again press RCL E.
Now try and convert back from the Cartesian to the Polar.
Shift
Rec( √ 2 , π ÷ 4 )

11. Modulus-Argument form of a complex number
Now we can define the Modulus-Argument form of a complex number to be:
z = r (cosθ + j sinθ)
Here r = |z| and θ = arg z
When writing a complex number in Modulus-Argument form it can be helpful to draw a diagram.
Remember that θ will take values between -π and π.

12. Modulus-Argument form of a complex number
Write the following numbers in Modulus-Argument form:
i) j ii) √3 – j iii) -4 – 4j
Solutions
i) 13(cos j sin 1.176)
ii) 2(cos (-π/6) + j sin (-π/6))
iii) 4√2(cos(-3π/4) + j sin(-3π/4))

13. Modulus-Argument form of a complex number
When writing a complex number in Modulus-Argument form it is important that it is written in the exact format as above, not:
z = -r (cosθ - j sinθ)
(r must be positive)
You can use the following trig identities to ensure that z is written in the correct form.
cos(π-θ) = -cosθ sin(π-θ) = sinθ
cos(θ-π) = -cosθ sin(θ-π) = -sinθ
cos(-θ) = cosθ sin(-θ) = -sinθ

14. Modulus-Argument form of a complex number
Example: Re-write -4(cosθ + j sinθ) in Modulus-Argument form:
-4(cosθ + j sinθ) = 4(-cosθ - j sinθ)
= 4(cos(θ-π) + j sin(θ-π))
This is now in Modulus-Argument form.
The Modulus is 4 and the Argument is (θ-π).
This may seem a bit confusing, however if you draw a diagram and tackle the problems like we did a few slides ago it makes more sense.
Now do Ex 2E pg 66

15. Sets of points using the Modulus-Argument form
What do you think arg(z1-z2) represents?
If z1 = x1 +y1j & z2 = x2 +y2j
Then z2 – z1 = (x2 – x1) + (y2 – y1)j
Now arg(z2 – z1) = inv tan ((y2 – y1)/(x2 – x1))
This represents the angle between the line from z1 to z2 and a line parallel to the real axis.
So arg(z2 – z1) = α Im
(x2,y2)
y2- y1 α
(x1,y1)
x2- x1 Re

16. Questions Draw Argand diagrams showing the sets of points z for which
i) arg z = π/3
ii) arg (z - 2) = π/3
iii) 0 ≤ arg (z – 2) ≤ π/3
Now do Ex 2F page 68 Im Re
π/3 Im Re
π/3 Im Re
π/3

17. History of complex numbers
In the quadratic formula (b2 – 4ac) is known as the discriminant.
If this is greater than zero the quadratic will have 2 distinct solutions.
If it is equal to zero then the quadratic will have 1 repeated root.
If it is less than zero then there are no solutions.
Complex numbers where invented so that we could solve quadratic equations whose discriminant is negative.
This can be extended to solve equations of higher order like cubic and quartic.
In fact complex numbers can be used to find the roots of any polyniomial of degree n.

18. History of complex numbers
In 1629 Albert Girard stated that an nth degree polynomial will have n roots, complex or real. Taking into account repeated roots.
For example the fifth order equation
(z – 2)(z-4)2(z2+9) = 0 has five roots.
2, 4(twice) 3j and -3j.
For any polynomial with real coefficients solutions will always have complex conjugate pairs.
Many great mathematicians have tried to prove the above.
Gauss achieved it in 1799.

19. Complex numbers and equations
Given that 1 + 2j is a root of 4z3 – 11z2 + 26z – 15 = 0
Find the other roots.
Since real coefficients, 1 – 2j must also be a root.
Now [z – (1 + 2j)] and [z – (1 – 2j)] will be factors.
So [z – (1 + 2j)][z – (1 – 2j)]
= [(z – 1) + 2j)][(z – 1) – 2j)]
= (z – 1)2 – (2j)2
= z2 – 2z + 1 – (-4)
= z2 – 2z + 5

20. Complex numbers and equations
You can now try to spot the remaining factors by comparing coefficients or you can use polynomial division.
4z – 3
z2 – 2z z3 – 11z z – 15 = 0
4z3 – 8z z
– 3z z – 15
Hence 4z3 – 11z2 + 26z – 15 = (z2 – 2z + 5)(4z – 3)
The roots are, 1 + 2j, 1 – 2j and 3/4

21. Complex numbers and equations
Given that -2 + j is a root of the equation
z4 + az3 + bz2 + 10z + 25 = 0
Find the values of a and b and solve the equation.
First you need to find the values of z4,z3 & z2 so that you can sub them in to the equation above.
z2 = (-2 + j)(-2 + j) = 3 – 4j
z3 = (3 – 4j)(-2 + j) = j
Z4 = ( j)(-2 + j) = j
Now
(-7 -24j) + a( j) + b(3 – 4j) + 10(-2 + j) +25 = 0