[discia] GCSE Maths › Algebra › Inequalities on Graphs › Practise
  1. Either paper 2 marks

    Here is an inequality.

    \(y \geq -3x + 2\)

    Does the point \((-5, 19)\) satisfy this inequality?

    You must show your working.

    Show answer
    Yes

    Put the \(x\)-coordinate of the point into the right-hand side and see what it comes to.

    \(-3 \times (-5) + 2 = 17\)

    The point has \(y = 19\), and \(19\) is greater than \(17\), so the statement \(19 \geq 17\) is true.

    Answer: Yes

    • M1substitute \(x = -5\): \(-3 \times (-5) + 2 = 17\)
    • A1Yes, comparing \(19\) with \(17\)
  2. Either paper 2 marks

    A region on a coordinate grid is drawn by ruling its boundary lines first. A boundary line is drawn solid when the points on the line itself belong to the region, and dashed when they do not.

    (a)

    A region is defined by \(y \gt 3x - 8\)

    Should the boundary line \(y = 3x - 8\) be drawn solid or dashed?

    (b)

    A different region is defined by \(x \lt 2\)

    Should the boundary line \(x = 2\) be drawn solid or dashed?

    Show answer
    (a) Dashed
    (b) Dashed

    A point on the line has \(y = 3x - 8\) exactly.

    The inequality is \(\gt\), which does not allow the two sides to be equal, so the points on the line are not part of the region.

    The boundary line is drawn dashed.

    The boundary is the vertical line through \(2\) on the \(x\)-axis.

    The inequality is \(\lt\), which does not allow \(x\) to equal \(2\), so the points on that vertical line are not part of the region.

    The boundary line is drawn dashed.

  3. Either paper 2 marks

    The line \(y = 2x + 5\) is drawn on the grid.

    xy-6-5-4-3-2123456-8-7-6-5-4-3-2123456780
    (a)

    State the inequality that is satisfied by every point below the line, but not by any point on the line itself.

    (b)

    State the inequality that is satisfied by every point above the line and by every point on the line.

    Show answer
    (a) \(y \lt 2x + 5\)
    (b) \(y \geq 2x + 5\)

    For any point on the line, \(y\) is exactly \(2x + 5\).

    A point below the line has the same \(x\) but a \(y\) that is lower down the grid than the value the line gives. The points on the line itself are not wanted, so the inequality is strict.

    \(y \lt 2x + 5\)

    This time the points on the line are wanted as well as the ones higher up the grid than it, so the inequality allows the two sides to be equal.

    \(y \geq 2x + 5\)

  4. Either paper 2 marks

    A point \((x, y)\) satisfies all three of these inequalities.

    \(x \gt 5 \qquad y \lt 4 \qquad x + y \lt 11\)

    Here are four points.

    \((6, 1) \qquad (4, 1) \qquad (6, 4) \qquad (8, 3)\)

    Exactly one of them satisfies all three inequalities.

    Write down that point.

    Show answer
    \((6, 1)\)

    Test each point against all three inequalities. A point only counts if it passes every one of them, so one failure is enough to rule it out.

    \((4, 1)\) fails \(x \gt 5\), because \(4\) is not more than \(5\).

    \((6, 4)\) fails \(y \lt 4\), because \(4\) is not less than \(4\).

    \((8, 3)\) fails \(x + y \lt 11\), because \(8 + 3 = 11\).

    \((6, 1)\) passes all three: \(6 \gt 5\), \(1 \lt 4\), and \(6 + 1 = 7\), which is less than \(11\).

    Answer: \((6, 1)\)

    • M1tests the points against the inequalities, e.g. \((4, 1)\) fails \(x \gt 5\)
    • A1\((6, 1)\) satisfies all three
  5. Either paper 2 marks

    The vertical line \(x = -2\) and the line \(y = -x - 1\) are both drawn on the grid.

    xy-6-5-4-3-2123456-8-7-6-5-4-3-2123456780

    Region \(R\) is made up of every point that lies to the left of the line \(x = -2\) and above the line \(y = -x - 1\). No point of \(R\) lies on either line.

    Write down the two inequalities that define \(R\).

    Show answer
    \(x \lt -2\) and \(y \gt -x - 1\)

    Take the two boundaries one at a time.

    The vertical line is \(x = -2\). Every point to the left of it has an \(x\)-coordinate less than \(-2\), so the first inequality is \(x \lt -2\).

    The sloped line is \(y = -x - 1\). A point above it has the same \(x\) but a \(y\) that is greater than the value the line gives, so the second inequality is \(y \gt -x - 1\).

    Neither line is part of \(R\), so both inequalities are strict — no \(\leq\) or \(\geq\).

    \(x \lt -2\) and \(y \gt -x - 1\)

    • B1\(x \lt -2\)
    • B1\(y \gt -x - 1\)
  6. Either paper 3 marks

    \(x\) and \(y\) are integers.

    \(x \geq 1 \qquad y \geq -1 \qquad x + y \leq 3\)

    (a)

    Work out how many points \((x, y)\) satisfy all three inequalities.

    (b)

    Write down the point that has the greatest \(x\)-coordinate.

    Show answer
    (a) \(10\)
    (b) \((4, -1)\)

    Work along the possible values of \(x\), smallest first.

    When \(x = 1\), the third inequality gives \(y \leq 3 - 1 = 2\), and the second gives \(y \geq -1\). That is \(4\) whole numbers.

    Every time \(x\) goes up by one there is one fewer value of \(y\) left, so the counts are \(4\), then \(3\), and so on down to \(1\).

    Adding them up gives \(10\) points.

    Pushing \(x\) as far right as possible leaves \(y\) as small as it can be, and the smallest \(y\) is allowed to be is \(-1\).

    Put \(y = -1\) into \(x + y \leq 3\):

    \(x \leq 3 - (-1) = 4\)

    So the point is \((4, -1)\).

    • (a) M1\(4\) values of \(y\) when \(x = 1\), one fewer each time \(x\) increases
    • (a) A1total \(10\) points
  7. Either paper 4 marks

    The three unlabelled points on the grid are the corners of a triangle, and the points \(P\) and \(Q\) lie on the sides of this triangle. The region \(S\) is defined by these three inequalities.

    (1) \(x \gt 0\)

    (2) \(y \gt -2\)

    (3) \(2x + y \leq 4\)

    xy-4-3-2-11234567-4-3-2-112345670PQ
    (a)

    Mina says that the point \(P\) with coordinates \((0, 3)\) does satisfy all the inequalities.

    Is Mina correct? You must give a reason for your answer.

    (b)

    Geoffrey says that the point \(Q\) with coordinates \((2, -2)\) does not satisfy all the inequalities.

    Is Geoffrey correct? You must give a reason for your answer.

    Show answer
    (a) No. At \((0, 3)\) inequality (1) has left-hand side \(0\), and it does not allow the two sides to be equal, so the point fails it and does not satisfy all three inequalities.
    (b) Yes. At \((2, -2)\) inequality (2) has left-hand side \(-2\), and it does not allow the two sides to be equal, so the point fails it and does not satisfy all three inequalities.

    Substitute \(x = 0\) and \(y = 3\) into each inequality.

    (1) \(0 \gt 0\) is false

    (2) \(3 \gt -2\) is true

    (3) \(2x + y = 3\) when \(x = 0\) and \(y = 3\), so \(3 \leq 4\) is true

    The point is on the edge given by inequality (1), where the two sides are equal, and that inequality does not allow equality. The point does not satisfy all three inequalities.

    Answer: No.

    Substitute \(x = 2\) and \(y = -2\) into each inequality.

    (1) \(2 \gt 0\) is true

    (2) \(-2 \gt -2\) is false

    (3) \(2x + y = 2\) when \(x = 2\) and \(y = -2\), so \(2 \leq 4\) is true

    The point is on the edge given by inequality (2), where the two sides are equal, and that inequality does not allow equality. The point does not satisfy all three inequalities.

    Answer: Yes.

    • (a) M1substitutes \((0, 3)\) into the inequalities, finding inequality (1) gives \(0\) exactly
    • (a) A1No, because inequality (1) does not allow equality
    • (b) M1substitutes \((2, -2)\) into the inequalities, finding inequality (2) gives \(-2\) exactly
    • (b) A1Yes, because inequality (2) does not allow equality
  8. Either paper 2 marks

    On the grid, show the region that satisfies \(y ≥ -2x + 1\)

    Draw the boundary line, then mark a point inside the region with the letter R.

    xy-5-4-3-212345-8-7-6-5-4-3-2123456780

    You cannot draw on this page. Draw this on paper, then show the answer and check your drawing against the model.

    Show answer
    The line \(y = -2x + 1\) drawn, and the region above it marked
    xy-5-4-3-212345-8-7-6-5-4-3-2123456780

    The boundary of the region is the line \(y = -2x + 1\). It crosses the \(y\)-axis at \((0, 1)\) and passes through \((1, -1)\). The inequality allows "equal to", so the line is part of the region and is drawn solid.

    To choose the side, test a point that is not on the line. Take \((0, 2)\): its \(y\)-coordinate is \(2\), and on the line at \(x = 0\) the value is \(1\).

    \(2 ≥ 1\) is true, so \((0, 2)\) is in the region. It is above the line, and so is the whole region.

    The region below the line is the one that is not wanted.

    • B1the line \(y = -2x + 1\) drawn
    • B1the region above the line marked
  9. Either paper 2 marks

    Joe draws this graph to identify the region \(R\) represented by

    \(y ≤ x - 2\)

    \(y ≤ -2 - x\)

    \(x < 3\)

    xy-8-6-4-22468-10-9-8-7-6-5-4-3-2123456789100

    Joe has shaded the region \(R\).

    Make two criticisms of his graph.

    Show answer
    (1) The line for the first inequality is drawn dashed, but it should be solid because the inequality includes equals. (2) The shading is on the wrong side of the line for the second inequality: the region should be below the line.

    Check each line of the graph against its own inequality, three things at a time: the style of the line, the side that is shaded, and where the line crosses the axis.

    First inequality, \(y ≤ x - 2\). The boundary is \(y = x - 2\), which crosses the \(y\)-axis at \(-2\). Joe's line crosses at \(-2\): correct. The sign includes equals, which keeps the points on the line, so the line should be solid; Joe drew it dashed: a fault. The sign asks for the points below the line; the shading is below Joe's line: correct.

    Second inequality, \(y ≤ -2 - x\). The boundary is \(y = -2 - x\), which crosses the \(y\)-axis at \(-2\). Joe's line crosses at \(-2\): correct. The sign includes equals, which keeps the points on the line, so the line should be solid; Joe drew it solid: correct. The sign asks for the points below the line; the shading is above Joe's line: a fault.

    Third inequality, \(x < 3\). The boundary is the vertical line \(x = 3\). Joe's vertical line is at \(x = 3\): correct. The sign is strict, which leaves out the points on the line, so the line should be dashed; Joe drew it dashed: correct. The sign asks for the points to the left of the line; the shading is to the left of Joe's line: correct.

    Only two lines have a fault, so the two criticisms are:

    (1) The line for the first inequality is drawn dashed, but it should be solid because the inequality includes equals.

    (2) The shading is on the wrong side of the line for the second inequality: the region should be below the line.

    • B1one fault named with its line: The line for the first inequality is drawn dashed, but it should be solid because the inequality includes equals
    • B1a second fault, on a different line: The shading is on the wrong side of the line for the second inequality: the region should be below the line
  10. Either paper 4 marks

    The region \(R\) is shown on the grid. It is bounded by four straight lines, and the points on these lines are part of \(R\).

    xy-6-5-4-3-2123456-8-7-6-5-4-3-2123456780R

    Find the four inequalities that define the region \(R\).

    Show answer
    \(x \geq -4\), \(y \geq -2\), \(y \leq 4\) and \(y \geq 3x + 1\)

    Take the three lines that are parallel to an axis first. Each one is a straight read from the grid.

    The region lies to the right of the line \(x = -4\), so \(x \geq -4\).

    The region lies above the line \(y = -2\), so \(y \geq -2\).

    The region lies below the line \(y = 4\), so \(y \leq 4\).

    The fourth line slopes and is not labelled, so find its equation from two points on it. Two points on the line are \((-1, -2)\) and \((1, 4)\).

    Going from \((-1, -2)\) to \((1, 4)\), \(y\) changes by \(6\) while \(x\) changes by \(2\), so the gradient is \(\frac{ 6 }{ 2 } = 3\).

    The line is \(y = 3x + c\). It passes through \((-1, -2)\), so \(-2 = -3 + c\) and \(c = -2 + 3 = 1\).

    So the line is \(y = 3x + 1\). To find which side of it \(R\) is on, test the corner \((-4, -2)\) of the region: the line gives \(y = -11\) at \(x = -4\), and \(-2\) is greater than \(-11\), so \(R\) is above the line and \(y \geq 3x + 1\).

    The four inequalities are \(x \geq -4\), \(y \geq -2\), \(y \leq 4\) and \(y \geq 3x + 1\).

    • B1\(x \geq -4\)
    • B1\(y \geq -2\)
    • B1\(y \leq 4\)
    • A1sloping line \(y \geq 3x + 1\), with gradient \(3\), intercept \(1\) and the correct direction