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.

  1. Example 1 · Solving a quadratic by null factor law

    Solve x2−5x+6=0x^2 - 5x + 6 = 0.

    Show worked solution

    Key idea: Factorise the quadratic into two brackets, then use the null factor law: if a×b=0a \times b = 0, then a=0a = 0 or b=0b = 0.

    1. Find two integers that multiply to 66 and add to −5-5: these are −2-2 and −3-3.
    2. Factorise: (x−2)(x−3)=0(x - 2)(x - 3) = 0
    3. Apply the null factor law:
      x−2=0 or x−3=0x - 2 = 0 \text{ or } x - 3 = 0
    4. Solve each equation:
      x=2 or x=3x = 2 \text{ or } x = 3
    5. Check by substituting back: (2)2−5(2)+6=4−10+6=0(2)^2 - 5(2) + 6 = 4 - 10 + 6 = 0 ✓ and (3)2−5(3)+6=9−15+6=0(3)^2 - 5(3) + 6 = 9 - 15 + 6 = 0 ✓

    Answer: x=2x = 2 or x=3x = 3

  2. Example 2 · Describing transformations of a parabola

    The graph of y=x2y = x^2 is transformed to give y=−3x2+4y = -3x^2 + 4. Describe the transformations applied to the standard parabola, state whether the new parabola opens upward or downward, give its vertex, and find the yy-intercept.

    Show worked solution

    Key idea: In y=ax2+cy = ax^2 + c, the value of aa controls the stretch and reflection, and cc shifts the graph vertically.

    1. Compare with the standard form y=ax2+cy = ax^2 + c: here a=−3a = -3 and c=4c = 4.
    2. Reflection: Because a=−3<0a = -3 < 0, the parabola is reflected in the xx-axis (opens downward).
    3. Vertical stretch: ∣a∣=3>1\lvert a \rvert = 3 > 1, so the parabola is narrower than y=x2y = x^2 by a factor of 33.
    4. Vertical translation: c=4c = 4, so the graph is shifted 44 units upward.
    5. The vertex is at (0, 4)(0,\ 4) and the yy-intercept is also (0, 4)(0,\ 4).

    Answer: Reflected in the xx-axis, stretched vertically by factor 33, shifted 44 units up; opens downward; vertex and yy-intercept at (0,4)(0, 4).

  3. Example 3 · Setting up and solving a quadratic from a worded problem

    A rectangular garden has a length that is 55 metres more than its width. The area of the garden is 3636 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.

    1. Let the width be xx metres. Then the length is (x+5)(x + 5) metres.
    2. Set up the area equation:
      x(x+5)=36x(x + 5) = 36
    3. Expand and rearrange:
      x2+5x−36=0x^2 + 5x - 36 = 0
    4. Factorise: find two integers that multiply to −36-36 and add to 55: these are 99 and −4-4.
      (x+9)(x−4)=0(x + 9)(x - 4) = 0
    5. Apply the null factor law: x=−9x = -9 or x=4x = 4. Since width must be positive, x=4x = 4.
    6. Width =4= 4 m, Length =4+5=9= 4 + 5 = 9 m. Check: 4×9=364 \times 9 = 36 ✓

    Answer: The garden is 44 metres wide and 99 metres long.

Common mistakes

Where students usually lose marks

Forgetting to set the equation equal to zero before factorising

x2−x=6⇒x(x−1)=6⇒x=6x^2 - x = 6 \Rightarrow x(x-1) = 6 \Rightarrow x = 6 or x−1=6x - 1 = 6

The null factor law only works when one side equals zero. Always rearrange first to get x2−x−6=0x^2 - x - 6 = 0, then factorise to get (x−3)(x+2)=0(x-3)(x+2) = 0.

Giving only one solution when there are two

(x−4)(x+1)=0⇒x=4(x-4)(x+1) = 0 \Rightarrow x = 4 (forgetting x=−1x = -1)

Each bracket gives its own solution. Always write x−4=0x - 4 = 0 or x+1=0x + 1 = 0 and solve both equations separately.

Getting the sign wrong when reading roots from factorised form

(x−3)(x+5)=0⇒x=3(x - 3)(x + 5) = 0 \Rightarrow x = 3 or x=5x = 5

Set each bracket equal to zero and solve: x−3=0x - 3 = 0 gives x=3x = 3, and x+5=0x + 5 = 0 gives x=−5x = -5 (not +5+5). The sign in the bracket flips when you solve.

Misreading the effect of a on the parabola's width

"y=5x2y = 5x^2 is wider than y=x2y = x^2 because 55 is a big number."

A larger positive value of aa makes the parabola narrower, not wider. The graph is steeper because yy values grow faster. A value of aa between 00 and 11 makes the parabola wider.

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Your turn

Solve x2−7x+10=0x^2 - 7x + 10 = 0 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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