Year 9 - Quadratics - Free practice
Year 9 Quadratics Worksheets & Practice Tests
Graphing parabolas, null factor law, solving quadratic equations.
Quick answer
How do you solve quadratic equations using the null factor law?
A quadratic equation is solved by the null factor law when you factorise it so that two brackets multiply to zero, then set each bracket equal to zero. For example, x² + x − 6 = 0 becomes (x + 3)(x − 2) = 0, giving x = −3 or x = 2. The solutions are also called the roots of the equation.
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Worked examples
Year 9 quadratics: worked examples
Three examples, from a standard question to a worded problem. Have a go at each one before you open the solution.
Example 1 · Solving a quadratic by null factor law
Solve .
Show worked solution
Key idea: Factorise the quadratic into two brackets, then use the null factor law: if , then or .
- Find two integers that multiply to and add to : these are and .
- Factorise:
- Apply the null factor law:
- Solve each equation:
- Check by substituting back: ✓ and ✓
Answer: or
Example 2 · Describing transformations of a parabola
The graph of is transformed to give . Describe the transformations applied to the standard parabola, state whether the new parabola opens upward or downward, give its vertex, and find the -intercept.
Show worked solution
Key idea: In , the value of controls the stretch and reflection, and shifts the graph vertically.
- Compare with the standard form : here and .
- Reflection: Because , the parabola is reflected in the -axis (opens downward).
- Vertical stretch: , so the parabola is narrower than by a factor of .
- Vertical translation: , so the graph is shifted units upward.
- The vertex is at and the -intercept is also .
Answer: Reflected in the -axis, stretched vertically by factor , shifted units up; opens downward; vertex and -intercept at .
Example 3 · Setting up and solving a quadratic from a worded problem
A rectangular garden has a length that is metres more than its width. The area of the garden is square metres. Find the dimensions of the garden.
Show worked solution
Key idea: Write an equation for the area of the rectangle, rearrange to equal zero, then factorise and apply the null factor law. Reject any negative solution since a length must be positive.
- Let the width be metres. Then the length is metres.
- Set up the area equation:
- Expand and rearrange:
- Factorise: find two integers that multiply to and add to : these are and .
- Apply the null factor law: or . Since width must be positive, .
- Width m, Length m. Check: ✓
Answer: The garden is metres wide and metres long.
Common mistakes
Where students usually lose marks
Forgetting to set the equation equal to zero before factorising
or
The null factor law only works when one side equals zero. Always rearrange first to get , then factorise to get .
Giving only one solution when there are two
(forgetting )
Each bracket gives its own solution. Always write or and solve both equations separately.
Getting the sign wrong when reading roots from factorised form
or
Set each bracket equal to zero and solve: gives , and gives (not ). The sign in the bracket flips when you solve.
Misreading the effect of a on the parabola's width
" is wider than because is a big number."
A larger positive value of makes the parabola narrower, not wider. The graph is steeper because values grow faster. A value of between and makes the parabola wider.
Try one yourself
Your turn
Solve using the null factor law. Enter the two solutions as a pair of numbers.
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About this worksheet
Why we built it
Year 9 is when students meet quadratics properly. These worksheets give your student varied practice across graphing parabolas, using the null factor law, and solving monic quadratics with integer roots - the exact skills tested at the Year 9 level.
What's covered
Sub-skills your student will practise
- ✓Recognising the standard parabola y = x^2
- ✓Effect of transformations on a parabola (y = ax^2 + c)
- ✓Solving quadratic equations by null factor law
- ✓Reading roots from a graph
- ✓Setting up a quadratic from a worded scenario
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Frequently asked
Questions parents ask about quadratics
What's a quadratic function?
A function where the highest power of x is 2 - so y = x^2, y = x^2 + 3x, y = 2x^2 - 5x + 1. Their graphs are always parabolas (U-shaped or upside-down U).
What's the null factor law?
If two things multiply to zero, at least one of them must be zero. So if (x - 3)(x + 5) = 0, then either x - 3 = 0 or x + 5 = 0. This is the main method Year 9 uses to solve quadratics.
How do students sketch a parabola?
Find the y-intercept (set x = 0), the x-intercepts if they exist (set y = 0 and solve), and the vertex (the turning point). Mark those points and draw a smooth curve.
What kind of quadratics does Year 9 solve?
Monic quadratics (where the x^2 coefficient is 1) with integer roots, using null factor law after factorising. Quadratic formula and completing the square arrive in Year 10.
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