Example – Solving simultaneous equations graphically

Solve these simultaneous equations using a graph.

y = 2 x

Plot the points for the two lines on the same graph.

The lines cross at the point (1.5, 3).

Unit 5 Using graphs to solve simultaneous equations Band h

Practising skills

1 a Write down the co-ordinates of the point of intersection of the 2 lines on this graph.

b Write down the solution of these equations.

x

⁺

y

^{= 4}

y

⁼

x

^{– 2}

c Check your answer by substituting values for

x

and

y

in the 2 equations.

2 a Write down the co-ordinates of the point of intersection of the 2 lines on this graph.

b Solve these equations in 2 ways.

y

^{= 4}

x

^{– 3}

y

^{= 3}

x

^{+ 1}

i By looking at the graph.

ii By algebra, using the substitution method.

3 a One of the lines on this graph is

y = x

and the other is 3

y

= 4

x

– 1. Which is which?

b Write down the co-ordinates of the point of intersection of the 2 lines.

c Solve these simultaneous equations.

y

x

y

^{= 4}

x

^{– 1}

i By looking at the graph.

ii By algebra, using the substitution method.

−1−1

4 a One of the lines on this graph has equation

y

= 13

x

+ 123.

The other has equation

y

^{= – 1}₂

x

^{+ 21}₂^.

Which is which?

b Write down the co-ordinates of the point of intersection of the 2 lines.

c Write down the 2 simultaneous equations for which this point is the solution.

d Check your solution by substituting your

x

^{- and}

y

-values in both equations.

5 a Copy and complete this table of values for

y

^{= 3}

x

^{– 2.} e Use algebra to check your solution.

6 a Draw the lines

y

⁼

x

^{+ 3 and}

y

^{= 1}₄

x

^{+ 21}₄ on the same graph.

e Check your answer using algebra to solve the equations.

8 a Draw the lines

y

^{= 3}

x

^{– 1 and}

y

⁼

x

– 1 on the same graph.

b Use your graph to solve the simultaneous equations

y

= 3

x

– 1 and

y

x

– 1.

c Use algebra to check your answer.

−1−1

Unit 5 Using graphs to solve simultaneous equations Band h

Developing fl uency

1 a Use algebra to solve these equations.

y

^{= 4}

x y

= 2

x

b Draw the lines

y

^{= 4 –}

x

^and

y

^{= 2}

x

– 2 on the same graph.

c Use your graph to check your answer to part a.

2 a Use algebra to solve these equations.

y

= 4

x

– 3

y

x

+ 3

b Draw the lines

y

^{= 4}

x

^{– 3 and}

y

⁼

x

^{+ 3}on the same graph.

c Use your graph to check your answers to part a.

d Check your answer again, this time by substituting the

x

^{- and}

y

-values in both equations.

3 a Use a graph to solve the simultaneous equations

y

^{= 2}

x

^{+ 3 and}

x

⁺

y

^{= 6.}

b Check your answer by solving these simultaneous equations by elimination.

–2

x

⁺

y

^{= 3}

x

⁺

y

^{= 6}

4 a Try to use algebra to solve these simultaneous equations.

y

^{= 2}

x

^{+ 3}

y

^{= 2}

x

^{– 1}

b What do you notice?

c Draw these two lines on the same axis.

d Use your graph to explain the result in part a.

5 a Draw the lines 2

x

+ 3

y

= 24 and

y

x

+ 3 on the same graph.

b Use your graph to solve the simultaneous equations 2

x

^{+ 3}

y

^{= 24 and}

y

⁼

x

^{+ 3.}

c Check your answer by solving the equations algebraically.

d Find the area of the region bounded by the lines 2

x

^{+ 3}

y

^{= 24,}

y

⁼

x

+ 3 and the

x

^and

y

^axes.

6 Two electricity companies advertise the following rates.

Green Power

Standing charge 30p per day Cost of electricity 18p per unit

Sparkle

Standing charge 50p per day Cost of electricity 16p per unit

a Write down an equation for the daily cost, C pence, of using

u

units of electricity for i Green Power ii Sparkle.

b On the same graph, draw two lines to illustrate the daily cost of electricity from each company for values of

u

from 0 to 15.

c i Use your graph to fi nd the number of units for which both companies charge the same amount. How much do they each charge for this number of units?

ii Check your answer to ci by solving your equations algebraically using the substitution method.

d The Watts family use an average of 12 units of electricity a day. Which company would you recommend?

7 A quadrilateral is bounded by these 4 lines 4

y

x

+ 24

y

^{+ 4}

x

^{= 23}

y

^{+ 4}

x

^{+ 11 = 0}

y

⁼

x

^{− 10}

a Draw the quadrilateral on a graph.

b Use your graph to fi nd the co-ordinates of the vertices of the quadrilateral.

c Show how you can check your answer to b by substituting the

x

and

y

co-ordinates into the appropriate pair of equations.

d What is the name of the quadrilateral?

Problem solving

1 Pete wants to hire a car. He wants to spend as little as possible.

He can hire it from one of two companies: U hire and Cars 2 go.

U hire 50p a mile

Cars 2 go

£60 plus 20p a mile

a Write down the equations for the cost £ C of hiring a car for

m

miles from each company.

b On the same graph draw lines for your equations.

Use the vertical axis for C with a scale from 0 to 200 and the horizontal axis for

m

with a scale from 0 to 400.

c Pete expects to drive 250 miles. Which company would you advise him to use?

2 Ros makes an accurate drawing of this diamond shape on graph paper.

The equations of the four lines are

y

^{= 3}

x

^{– 3}

y

^{= 3}

x

^{– 9}

y

^{= 3}

x

^{+ 15}

y

= 3

x

+ 9

a Make tables of values for these lines, taking values of

x

from 0 to 5.

b Draw a graph showing the parts of the lines that make the shape.

c Write down the co-ordinates of the four vertices, P, Q, R and S.

d Show how Ros could have used algebra to fi nd the co-ordinates of the vertices.

Use this method to check your answers to part c.

e What is the mathematical name for this shape?

Exam-styleExam-style

P Q

Unit 5 Using graphs to solve simultaneous equations Band h 3 Catherine draws a triangle which is bounded by the lines

x

+ 2

y

= 12, 2

y

x

+ 4 and

y

x

−3.

a Draw the three lines on the same graph.

b i Write the coordinates of the vertices of the triangle.

ii Use your diagram to solve the following pairs of simultaneous equations.

x

^{+ 2}

y

^{= 12, 2}

y

⁼

x

^{+ 4 B}

x

^{+ 2}

y

^{= 12,}

y

⁼

x

− 3 C 2

y

⁼

x

^{+ 4,}

y

⁼

x

^{− 3}

Catherine adds 2 horizontal and 2 vertical lines to her diagram to form a rectangle around her triangle. Each line passes through one of the vertices of the triangle.

c i Add these lines to your diagram.

ii State the equations of the lines that Catherine adds.

iii Find the area of the rectangle bounded by these 4 lines.

d Find the area of Catherine’s triangle.

4 Look at these pay-as-you-go tariffs for 2 mobile phone companies.

Q-Mobile

30p per minute for calls 8p per text

Pear

20p per minute for calls 12p per text

Let

m

stand for the number of call minutes and

t

stand for the number of texts.

a Write down an equation for

m

^and

t

for a bill of £5 with i Q-Mobile ii Pear.

b Draw a graph of your equations.

Chloe uses Q-Mobile and Daisy uses Pear.

c One week they use the same number of call minutes as each other, and they send the same number of texts. They both use exactly £5 of credit.

How many call minutes and texts do they each use?

d Another week both girls make 4 minutes of calls and sent 40 texts.

i Mark a point on your graph to show this.

Both girls have £5 credit.

ii Who has to buy more credit? How much more credit does she need?

iii Who has credit left over? How much?

5 Here is the graph of the circle

x

²⁺

y

²^{= 4.}

The line

y

^{= 2}

x

+ 1 cuts the circle at 2 points.

Find the co-ordinates of these 2 points.

Higher tier only

6 Here is the graph of the ellipse

x

9 +

y

4 = 1.

The line

x

y

=1 cuts the ellipse at two points.

Find the co-ordinates of these two points.

Higher tier only

c Check your solution by substituting the

x

^{- and}

y-

values in both equations.

2 a Use a graph to solve the simultaneous equations

y

^{= 3}

x

^{+ 4 and}

y

⁼

x

^{+ 2.}

b Check your answer algebraically by using the method of substitution.

c Check your answer again, this time substituting the

x

^and

y

values in both equations.

3 Avonford College compares the cost of two coach companies for a school trip.

Speedy Coaches

b On the same graph, draw 2 lines to illustrate the daily cost of hiring a coach from each company for values of

m

from 0 to 100.

c i Use your graph to fi nd the number of miles for which both fi rms charge the same amount. How much do they each charge for this number of miles?

ii Check your answer to ci by solving your equations algebraically using the substitution method.

d Avonford College are organising a coach for an 80-mile round trip.

i Which company would you recommend?

ii How much do Avonford College save by choosing this company?

Strand 5 Working with quadratics

Unit 1 Band h

Unit 4 Band j

Unit 2 Band h

Unit 3 Band j

Unit 7 Band j

Factorising quadratics

The quadratic formula

Solve equations by factorising

Factorising harder quadratics

Solving quadratic inequalities Page 166

Unit 5 Band j

Completing the square

Higher 2 Higher 2

Page 172

Unit 6 Band j

Simultaneous equations with quadratics

Higher 2 Higher 2

Higher 2

5 Unit 1 Factorising quadratics

In document Mathematics for Edexcel GCSE - Foundation 2-Higher 1 (2015) (Page 169-177)