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Cambridge 0580 IGCSE  ·  Topic E3.2

E3.2 Drawing Straight-Line Graphs

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01 — Explanation

How do I turn an equation into a table of values?

Choose a spread of xx-values (usually given in the question, e.g. x=−2x = -2 to 33), then substitute each into the equation to get the matching yy-value. For y=3x+2y = 3x + 2:

xx−2-2−1-100112233
yy−4-4−1-12255881111

02 — Explanation

How do I plot the line accurately?

Plot each (x,y)(x, y) pair as a point, then join them with a single, ruled, straight line extending across the full grid — not just dot-to-dot between the plotted points. If one plotted point doesn't sit on the same line as the rest, you've made an arithmetic error and should recheck that value.

03 — Explanation

What if the equation isn't already in the form y = \ldots?

Some equations are given implicitly, e.g. 3x+2y=53x + 2y = 5. Rearrange for yy first so you can substitute cleanly:

3x+2y=5  ⟹  2y=5−3x  ⟹  y=5−3x23x + 2y = 5 \implies 2y = 5 - 3x \implies y = \frac{5 - 3x}{2}

Then build the table exactly as before. Choosing xx-values that are multiples of the denominator (here, even numbers) often keeps the yy-values as whole numbers or simple fractions, which makes plotting far less error-prone.

04 — Explanation

What if no x-range is given to me at all?

If the question doesn't specify a range, pick values that keep the table manageable and centred near the origin — typically five or six integers such as x=−2,−1,0,1,2,3x = -2, -1, 0, 1, 2, 3. Avoid huge values that would force an awkwardly-scaled grid; the goal is a clear, evenly-spaced set of points that reveal the line's direction.

05 — Explanation

How do I recognise and draw a horizontal or vertical line?

An equation like y=4y = 4 has the same yy-value for every xx, so it's a horizontal line through 44 on the yy-axis — no table is even needed. Likewise x=−2x = -2 is a vertical line through −2-2 on the xx-axis. These are easy to lose marks on if you try to force them into a sloped-line table by mistake.

Exam tip

Examiners check plotted points to within half a small square. If your line doesn't pass through all your table values, don't force a "best fit" line through them — a straight-line graph must be exact, so go back and find your arithmetic mistake.

06 — Worked example

Worked example

Draw the graph of y=7−4xy = 7 - 4x for −1⩽x⩽3-1 \leqslant x \leqslant 3.

  1. Substitute each integer xx from −1-1 to 33:

x=−1:y=7−4(−1)=11x=-1: y = 7-4(-1) = 11

x=0:y=7x=0: y = 7

x=1:y=3x=1: y = 3

x=2:y=−1x=2: y = -1

x=3:y=−5x=3: y = -5

  1. Plot (−1,11),(0,7),(1,3),(2,−1),(3,−5)(-1,11), (0,7), (1,3), (2,-1), (3,-5).
  2. Draw one ruled straight line through all five points, extending slightly beyond the first and last point.

07 — Worked example

Worked example: an implicit equation

Draw the graph of 2x+3y=62x + 3y = 6 for −3⩽x⩽3-3 \leqslant x \leqslant 3.

  1. Rearrange for yy: 3y=6−2x  ⟹  y=6−2x33y = 6 - 2x \implies y = \dfrac{6-2x}{3}.
  2. Choose multiples of 33 to keep yy a whole number: x=−3:y=123=4x=-3: y = \dfrac{12}{3}=4; x=0:y=2x=0: y=2; x=3:y=0x=3: y=0.
  3. Plot (−3,4),(0,2),(3,0)(-3,4), (0,2), (3,0) and draw one ruled line through them, extending across the grid.