Number the equations.

$\begin{array}{rcl} x^{2} + y^{2} & = & 25 (1) \\ y - 3 x & = & 13 (2) \end{array}$

There is one quadratic equation and one linear equation so this must be done by substitution.

Rearrange equation (2) so that the subject is either $x$ or $y$ , $y$ will be the easiest to work with here.

$\begin{array}{rcl} y & = & 13 + 3 x (2) \end{array}$

Substitute this into equation (1) to eliminate $y$ .

$x^{2} + {(13 + 3 x)}^{2} = 25$

[1]

Expand the brackets, remember that a bracket squared should be treated the same as double brackets.

$x^{2} + (13 + 3 x) (13 + 3 x) = 25 x^{2} + {(13)}^{2} + 13 (3 x) + 13 (3 x) + {(3 x)}^{2} = 25$

Simplify.

$\begin{array}{rcl} x^{2} + 169 + 39 x + 39 x + 9 x^{2} & = & 25 \\ 10 x^{2} + 78 x + 169 & = & 25 \end{array}$

Correct expansion [1]

Rearrange to form a quadratic equation that is equal to zero.

$\begin{array}{rcl} 210 x^{2} + 78 x + 169 - 25 & = & 0 \\ 10 x^{2} + 78 x + 144 & = & 0 \end{array}$

[1]

This is a non-calculator paper and no degree of accuracy is asked for, so the quadratic will be possible to factorise.
Make the problem easier to deal with by dividing every term by 2.

$\begin{array}{rcl} 5 x^{2} + 39 x + 72 & = & 0 \end{array}$

We need a pair of numbers that multiply to $a c$ which in this case is $5 \times 72 = 360$ , and add to $b$ , which in this case is 39.

15 and 24 satisfy these conditions

Rewrite the middle term using 15 and 24.

$\begin{array}{rcl} 5 x^{2} + (15 + 24) x + 72 & = & 0 \\ 5 x^{2} + 15 x + 24 x + 72 & = & 0 \end{array}$

Factorise the first two terms, using $5 x$ as the highest common factor, and the second two terms using 24 as the factor.

$5 x (x + 3) + 24 (x + 3) = 0$

Note that these now have the same factor of $(x + 3)$ so this can be used as a common factor.

$(5 x + 24) (x + 3) = 0$

[1]

Once you get to this spot it may be worth expanding the brackets to check your answer.

When two numbers are multiplied and the result is zero, at least one of them must be zero, so we can say that

$5 x + 24 = 0 or x + 3 = 0$

Solving both of these equations to find the values for $x$ .

$\begin{array}{rcl} x_{1} & = & - \frac{24}{5} \\ x_{2} & = & - 3 \end{array}$

Substitute the values of $x$ into one of the equations (the linear equation is easier) to find the values of $y$ .
Leaving the value of $x_{1}$ as a fraction makes is easier to calculate with.

$y_{1} = 13 + 3 (- \frac{24}{5}) = 13 - \frac{72}{5} = 13 - 14 \frac{2}{5} = - 1 \frac{2}{5} y_{2} = 13 + 3 (- 3) = 13 - 9 = 4$

$x_{1} = - \frac{24}{5}, y_{1} = - \frac{7}{5}$
$x_{2} = - 3, y_{2} = 4$ [1]

Simultaneous Equations (OCR GCSE Maths)

Exam Questions

1. Number Operations & Integers

Number Toolkit

Prime Factors, HCF & LCM

2. Fractions, Decimals & Percentages

Fractions

Percentages

Working with FDP

3. Indices & Surds

Powers, Roots & Standard Form

Surds

4. Approximation & Estimation

Rounding, Estimation & Bounds

Using a Calculator

5. Ratio, Proportion & Rates of Change

Ratio Toolkit

Ratio Problem Solving

Direct & Inverse Proportion

Exchange Rates & Best Buys

Simple & Compound Interest, Growth & Decay

6. Algebra

Algebra Toolkit

Algebraic Roots & Indices

Expanding Brackets

Factorising

Completing the Square

Algebraic Fractions

Rearranging Formulae

Algebraic Proof

Linear Equations

Solving Quadratic Equations

Solving Inequalities

Simultaneous Equations

Iteration

Forming & Solving Equations

Functions

Sequences

7. Graphs of Equations & Functions

Coordinate Geometry

Linear Graphs y = mx + c

Graphing Inequalities

Graphs of Functions

Equation of a Circle

Estimating Gradients & Areas under Graphs

Real-Life Graphs

Transformations of Graphs

8. Basic Geometry

Geometry Toolkit

Angles in Polygons & Parallel Lines

Circle Theorems

9. Congruence & Similarity

Congruence, Similarity & Geometrical Proof

Area & Volume of Similar Shapes

Vectors

Transformations

10. Mensuration

Standard & Compound Units

Bearings, Scale Drawing, Constructions & Loci

Area & Perimeter

Circles, Arcs & Sectors

Volume & Surface Area

Right-Angled Triangles - Pythagoras & Trigonometry

Sine, Cosine Rule & Area of Triangles

3D Pythagoras & Trigonometry

11. Probability

Probability Toolkit

Simple Probability Diagrams

Tree Diagrams

Combined & Conditional Probability

12. Statistics

Statistics Toolkit

Statistical Diagrams

Histograms

Cumulative Frequency & Box Plots

Scatter Graphs & Correlation