Complex Numbers (CIE A Level Maths: Pure 3)

Exam Questions

3 hours36 questions
1a
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2 marks

z subscript 1 equals 3 plus i space and z subscript 2 equals negative 5 plus 2 i

(i)
Work out z subscript 1 plus z subscript 2 and z subscript 1 minus z subscript 2.
(ii)
Use your answers to part (i) to write down Re open parentheses z subscript 1 plus z subscript 2 close parentheses and Im open parentheses z subscript 1 minus z subscript 2 close parentheses.
1b
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2 marks

By first writing z subscript 1 cross times space z subscript 2 equals left parenthesis 3 plus i right parenthesis left parenthesis negative 5 plus 2 i right parenthesis,  and then expanding brackets and using the fact that  i squared equals negative 1,  show that 

z subscript 1 cross times z subscript 2 equals negative 17 plus i

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2a
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1 mark

Given that z subscript 1 equals 3 minus 2 i and z subscript 2 equals 4 plus 3 i :

Write down z subscript 1 superscript asterisk times and z subscript 2 superscript asterisk times

2b
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2 marks

Show that z subscript 1 cross times space z subscript 2 superscript asterisk times equals 6 minus 17 i and z subscript 2 cross times space z subscript 2 superscript asterisk times equals 25

2c
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1 mark

Explain why z subscript 1 over z subscript 2 equals fraction numerator z subscript 1 cross times z subscript 2 superscript asterisk times over denominator z subscript 2 cross times z subscript 2 superscript asterisk times end fraction

2d
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2 marks

Use the results of parts (b) and (c) to work out  z subscript 1 over z subscript 2,  giving your answer in the form  a plus b i  where a and b are real numbers.

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3
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2 marks

Given that z subscript 1 equals negative 7 plus 6 i comma space z subscript 2 equals a plus b i and z subscript 1 plus z subscript 2 equals 1 minus 3 i, find a and b where a and b are real numbers.

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4a
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2 marks

f left parenthesis z right parenthesis equals z squared minus 14 z plus 50

By first working out left parenthesis 7 minus i right parenthesis squared,  show that z equals 7 minus i is a solution to f left parenthesis z right parenthesis equals 0.

4b
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2 marks

Show that left parenthesis 7 minus i right parenthesis to the power of asterisk times is another solution to f left parenthesis z right parenthesis equals 0.

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5a
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1 mark

Using laws of surds and the fact that  i equals square root of negative 1 end root,  find both solutions to the equation  z squared plus 16 equals 0. Your answers should be given as multiples of  i.

5b
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5 marks

Solve the equation z squared minus 4 z plus 13 equals 0 for z

(i)
by using the quadratic formula
(ii)
by completing the square.

In each case, give your answers in the form a plus b i where a and b are real numbers. Confirm that the two methods give the same results.

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6
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4 marks

The two roots of a quadratic equation with real coefficients are z subscript 1 and z subscript 2

Given that  z subscript 2 equals 5 minus 3 i, and that non-real roots to quadratic equations with real coefficients always occur in complex conjugate pairs,

(i)
write down the other root of the equation
(ii)
by first writing the equation in the form left parenthesis z minus z subscript 1 right parenthesis left parenthesis z minus z subscript 2 right parenthesis equals 0, find the equation in the form  z squared plus b z plus c equals 0 where a and b are real constants.

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7a
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3 marks

Given that beta equals 1 plus 2 i,

(i)
express beta squared and beta cubed in the form a plus b i, where a and b are real numbers
(ii)
hence show that beta is a root of the cubic equation

z cubed plus z squared minus z plus 15 equals 0

7b
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4 marks

The other two roots of the cubic equation in part (a) (ii) are gamma and delta, where gamma is a non-real complex number and delta is a real number.

Given that non-real roots to cubic equations with real coefficients always occur in complex conjugate pairs,

(i)
write down gamma in the form a plus b i, where a and b are real numbers
(ii)
show that left parenthesis z minus beta right parenthesis left parenthesis z minus gamma right parenthesis equals z squared minus 2 z plus 5
(iii)
hence fully factorise z cubed plus z squared minus z plus 15 and determine the value of delta.

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8
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5 marks

f left parenthesis z right parenthesis equals z cubed minus 9 z squared plus c z plus d, where c and d are real numbers.

The roots of the equation f left parenthesis z right parenthesis equals 0 are z subscript 1 comma space z subscript 2 and z subscript 3.

Given that z subscript 1 equals 3 and z subscript 2 equals 3 minus i,  and that non-real roots to cubic equations with real coefficients always occur in complex conjugate pairs,

(i)
write down z subscript 3 in the form a plus b i, where a and b are real numbers
(ii)
by first writing   f left parenthesis z right parenthesis equals left parenthesis z minus z subscript 1 right parenthesis left parenthesis z minus z subscript 2 right parenthesis left parenthesis z minus z subscript 3 right parenthesis,  find the values of c and d.

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9a
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1 mark

Let w equals a plus b i, where a and b are real numbers, be a general complex number.

Show that left parenthesis negative w right parenthesis squared equals w squared.

9b
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3 marks

For a complex number z, a square root of z is a complex number x plus y i where x and y are real numbers and where

left parenthesis x plus y i right parenthesis squared equals z

(i)
Given that  z equals negative 5 minus 12 i, show that is 2 minus 3 i a square root of z.
(ii)
Use the results of part (a) to write down the other square root of z.

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1
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3 marks

Given that z subscript 1 equals 2 minus 3 i and  z subscript 2 equals negative 7 plus i, work out the following:

(i)
z subscript 1 cross times z subscript 2
(ii)
R e left parenthesis z subscript 1 plus z subscript 2 right parenthesis
(iii)
I m left parenthesis z subscript 2 minus z subscript 1 right parenthesis

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2a
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3 marks

Given that z subscript 1 equals 5 plus 7 i and z subscript 2 equals 2 minus i:

Work out  z subscript 1 cross times space z subscript 2 superscript asterisk times and z subscript 2 cross times z subscript 2 superscript asterisk times

2b
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2 marks

Hence or otherwise work out z subscript 1 over z subscript 2, giving your answer in the form a plus b i where a and b are real numbers.

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3
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3 marks

For a general complex number z equals x plus i y,  where x and y are real numbers, show that:

(i)
R e left parenthesis z right parenthesis equals fraction numerator z plus z to the power of asterisk times over denominator 2 end fraction

(ii)
I m left parenthesis z right parenthesis equals fraction numerator z minus z to the power of asterisk times over denominator 2 i end fraction

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4a
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2 marks

Given that z subscript 1 equals 3 plus a i comma space space z subscript 2 equals b minus 7 i and z subscript 1 plus z subscript 2 equals 14 plus i find a and b where a and b are real numbers.

4b
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3 marks

f left parenthesis z right parenthesis equals z squared minus 6 z plus 34.

(i)
Show that z equals 3 minus 5 i is a solution to f left parenthesis z right parenthesis equals 0.
(ii)
Write down the other solution to f left parenthesis z right parenthesis equals 0.

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5
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3 marks

Solve the following equations for z, giving your answers in the form a plus b i where a and b are real numbers:

(i)
z squared plus 13 equals 0
(ii)
z squared minus 14 z plus 53 equals 0

In each case, be sure to show your working.

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6
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4 marks

Given that 7 minus i is one of the roots of a quadratic equation with real coefficients,

(i)
write down the other root of the equation
(ii)
find the equation giving your answer in the form z squared plus b z plus c equals 0 where b and c are real constants.

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7a
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3 marks

Given that beta equals negative 2 minus 3 i,

(i)
express beta squared and beta cubed in the form a plus b i, where a and b are real numbers
(ii)
hence show that beta is a root of the cubic equation

z cubed minus z squared minus 7 z minus 65 equals 0

7b
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3 marks

Find the other two roots of the cubic equation in part (a) (ii), being sure to show clear algebraic working.

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8
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5 marks

f left parenthesis z right parenthesis equals z cubed minus 6 z squared plus c z plus d, where c and d are real numbers.

Given that negative 2 and 4 minus i are roots of the equation  f left parenthesis z right parenthesis equals 0,

(i)
write down the other complex root of the equation
(ii)
find the value of c and the value of d.

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9a
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3 marks

For a complex number z,  a square root of z is a complex number x plus i y where x and y are real numbers and where

left parenthesis x plus i y right parenthesis squared equals z

Given that z equals negative 3 plus 4 i, show that x squared minus y squared equals negative 3 and 2 x y equals 4.

9b
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4 marks
(i)
Solve the equations in part (a) to find the possible values of x and y.
(ii)
Hence determine the two square roots of z equals negative 3 plus 4 i.

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1
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6 marks

Given that z subscript 1 equals 1 minus 2 i and  z subscript 2 equals negative 3 plus 5 i, work out the following:

(i)
R e left parenthesis z subscript 2 minus z subscript 1 right parenthesis

(ii)
I m left parenthesis z subscript 1 z subscript 2 right parenthesis

(iii)
open parentheses z subscript 1 over z subscript 2 close parentheses to the power of asterisk times

For part (iii) give your answer in the form  a plus b i, where a and b are real numbers.

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2
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4 marks

The complex number z satisfies the equation left parenthesis 2 plus 5 i right parenthesis left parenthesis z plus 2 i right parenthesis equals negative 7 minus 32 i.

Find z, giving your answer in the form  a plus b i, where a and b are real numbers.

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3a
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4 marks

Given that z subscript 1 equals a minus 6 i comma z subscript 2 equals 1 plus b i and  z subscript 1 z subscript 2 equals negative 17 minus 9 i, where a and b are real numbers, find the possible values of a and b.

3b
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2 marks

Using your answers to part (a), write down values for c and d that will satisfy the equation

negative left parenthesis 3 plus i right parenthesis left parenthesis c plus d i right parenthesis equals negative 17 minus 9 i

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4
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3 marks

The equation  z squared plus b z plus 18 equals 0, where  b element of straight real numbers,  has distinct non-real complex roots. 

Find the range of possible values of  b.

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5
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4 marks

Given that negative 3 plus 2 i is one of the roots of the quadratic equation  z squared plus b z plus c equals 0,  where b and c are real constants, find the values of b and c.

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6
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4 marks

f left parenthesis z right parenthesis equals z squared plus 2 i z minus 10

(i)
Show that f left parenthesis z right parenthesis can be rewritten in the form  left parenthesis z plus a i right parenthesis squared plus b, where a and b are real numbers to be found.
(ii)
Hence find the solutions to the equation f left parenthesis z right parenthesis equals 0.

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7a
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3 marks

Show that alpha equals negative 1 plus 4 i  is a root of the cubic equation

z cubed plus 5 z squared plus 23 z plus 51 equals 0

7b
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3 marks

Find the other two roots of the cubic equation in part (a), being sure to show clear algebraic working.

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8
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5 marks

f left parenthesis z right parenthesis equals z cubed plus z squared plus c z plus d, where c and d are real numbers.

Given that 3  and negative 2 minus 3 i  are roots of the equation  f left parenthesis z right parenthesis equals 0, find the value of c and the value of d.

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9
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6 marks

For a complex number z,  a square root of z is a complex number x plus i y where x and y are real numbers and where

left parenthesis x plus i y right parenthesis squared equals z

By expanding left parenthesis x plus i y right parenthesis squared and solving the resultant equation for x and y, determine the two square roots of the complex number  z equals negative 21 minus 20 i.

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1
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3 marks

For a general complex number z equals x plus i y, where  x comma y element of straight real numbers  and z not equal to 0, show that

(i)
R e open parentheses 1 over z plus 1 over z to the power of asterisk times close parentheses equals fraction numerator 2 x over denominator x squared plus y squared end fraction

(ii)
I m open parentheses 1 over z plus 1 over z to the power of asterisk times close parentheses equals 0

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2
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3 marks

Given that z subscript 1 equals 3 minus 4 i comma space space z subscript 2 equals a plus b i and fraction numerator 2 z subscript 1 over denominator z subscript 2 end fraction equals negative 1 minus 7 i,  where a and b are real numbers, find the values of a and b.

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3
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4 marks

Find all the complex numbers z for which  z squared equals 4 z to the power of asterisk times.

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4
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4 marks

alpha comma space beta space and gamma are the three roots of the cubic equation

z cubed plus left parenthesis b minus 3 right parenthesis z squared plus left parenthesis 7 minus 3 b right parenthesis z minus 21 equals 0

 where  b element of straight real numbers.

Given that alpha equals 3 and that beta and gamma are distinct non-real complex numbers, find the range of possible values of  b.

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5
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4 marks

Given that 3 plus q i is one of the roots of the quadratic equation  z squared minus 12 p z plus 58 equals 0,  where p and q are positive real constants, find the values of p and q.

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6
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4 marks

Work out the solutions to the equation  z squared minus 6 i z minus 14 equals 0.  Be sure to show clear algebraic working.

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7a
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5 marks

You are given that the complex number alpha equals 1 minus 3 i satisfies the cubic equation

z cubed plus 4 z squared plus k z plus m equals 0 comma

where k and m are real constants.

By first calculating alpha squared and  alpha cubed,  find the values of k and m.

7b
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3 marks

Find the other two roots of the cubic equation.

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8a
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4 marks

f left parenthesis z right parenthesis equals z to the power of 4 minus 2 z cubed plus 6 z squared plus p z plus 125, where p is a real constant.

Given that 3 minus 4 i is a root of the equation f left parenthesis z right parenthesis equals 0 , show that z squared plus 4 z plus 5 is a factor of f left parenthesis z right parenthesis.

8b
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3 marks

Hence find the value of p and solve completely the equation  f left parenthesis z right parenthesis equals 0.

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9a
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5 marks

The principal square root of a complex number z is defined as

square root of z equals x plus i y

where  and  are real numbers and x greater or equal than 0.  If x equals 0 then the value for  is chosen such that  y greater or equal than 0.  Note that the other square root of  will then be given by negative square root of z equals negative x minus i y.

Show that

x equals square root of fraction numerator R e left parenthesis z right parenthesis plus square root of left parenthesis R e left parenthesis z right parenthesis right parenthesis squared plus left parenthesis I m left parenthesis z right parenthesis right parenthesis squared end root over denominator 2 end fraction end root

9b
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2 marks

Given that x greater than 0, derive a formula for y in terms of x and I m left parenthesis z right parenthesis, and explain why y in this case will always have the same sign (positive, negative, or zero) as I m left parenthesis z right parenthesis.

9c
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2 marks

Hence show that in general

y equals plus-or-minus square root of fraction numerator negative R e left parenthesis z right parenthesis plus square root of left parenthesis R e left parenthesis z right parenthesis right parenthesis squared plus left parenthesis I m left parenthesis z right parenthesis right parenthesis squared end root over denominator 2 end fraction end root

with the choice of the positive or negative value being dependent on the properties of z.

9d
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3 marks

Explain what must be true of z for each of the following to be true:

(i)
x equals 0 comma space y not equal to 0
(ii)
x not equal to 0 comma space y equals 0
(iii)
x equals 0 comma space y equals 0

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