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Modulus & Argument (CIE A Level Maths: Pure 3)
Revision Note
Modulus & Argument
How do I find the modulus of a complex number?
- The modulus of a complex number is its distance from the origin when plotted on an Argand diagram
- The modulus of is written
- If , then we can use Pythagoras to show…
- A modulus is always positive
- the modulus is related to the complex conjugate by…
- This is because
- In general,
- e.g. both and have a modulus of 5, but simplifies to which has a modulus of 8
How do I find the argument of a complex number?
- The argument of a complex number is the anti-clockwise angle that it makes when starting at the positive real axis on an Argand diagram
- Arguments are measured in radians
- Sometimes these can be given exact in terms of
- The argument of is written
- Arguments can be calculated using right-angled trigonometry
- This involves using the tan ratio plus a sketch to decide whether it is positive/negative and acute/obtuse
- Arguments are usually given in the range
- Negative arguments are for complex numbers in the third and fourth quadrants
- Occasionally you could be asked to give arguments in the range
- The argument of zero, is undefined (no angle can be drawn)
Worked example
Examiner Tip
- Give non-exact arguments in radians to 3 significant figures.
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Modulus-Argument (Polar) Form
The complex number is said to be in Cartesian form. There are, however, other ways to write a complex number, such as in modulus-argument (polar) form.
How do I write a complex number in modulus-argument (polar) form?
- The Cartesian form of a complex number, , is written in terms of its real part, , and its imaginary part,
- If we let and , then it is possible to write a complex number in terms of its modulus, , and its argument, , called the modulus-argument (polar) form, given by...
- It is usual to give arguments in the range
- Negative arguments should be shown clearly, e.g. without simplifying to either or
- Occasionally you could be asked to give arguments in the range
- If a complex number is given in the form , then it is not currently in modulus-argument (polar) form due to the minus sign, but can be converted as follows…
- By considering transformations of trigonometric functions, we see that and
- Therefore can be written as , now in the correct form and indicating an argument of
- To convert from modulus-argument (polar) form back to Cartesian form, evaluate the real and imaginary parts
- E.g. becomes
What are the rules for moduli and arguments under multiplication and division?
- When two complex numbers, and , are multiplied to give , their moduli are also multiplied
- When two complex numbers, and , are divided to give , their moduli are also divided
- When two complex numbers, and , are multiplied to give , their arguments are added
- When two complex numbers, and , are divided to give , their arguments are subtracted
How do I multiply complex numbers in modulus-argument (polar) form?
- The main benefit of writing complex numbers in modulus-argument (polar) form is that they multiply and divide very easily (often quicker than when in Cartesian form)
- To multiply two complex numbers, and , in modulus-argument (polar) form we use the rules from above to multiply their moduli and add their arguments
- So if and then the rules above give…
- Sometimes the new argument, , does not lie in the range (or if this is being used)
- An out-of-range argument can be adjusted by either adding or subtracting
- E.g. If and then
- This is currently not in the range , but by subtracting from to give , a new argument is formed that lies in the correct range and represents the same angle on an Argand diagram
- The rules of multiplying the moduli and adding the arguments can also be applied when…
- …multiplying three complex numbers together, , or more
- …finding powers of a complex number (e.g. can be written as )
- Whilst not examinable, the rules for multiplication can be proved algebraically by multiplying by , expanding the brackets and using compound angle formulae
How do I divide complex numbers in modulus-argument (polar) form?
- To divide two complex numbers, and in modulus-argument (polar) form, we use the rules from above to divide their moduli and subtract their arguments
- So if and then the rules above give…
- As with multiplication, sometimes the new argument, , can lie out of the range (or the range if this is being used)
- You can add or subtract to bring out-of-range arguments back in range
- Whilst not examinable, the rules for division can be proved algebraically by dividing by , using complex division and compound angle formulae
Worked example
Examiner Tip
- The rules for multiplying and dividing in modulus-argument (polar) form must be learnt (they are not given in the formula booklet).
- Remember to add or subtract to any out-of-range arguments to bring them back in range.
- If a question does not give a clear range for arguments, then both or would be accepted.
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