Equations of Straight Line Graphs

Students learn how to plot and derive the equation of straight line graphs in the form y = mx + c.  Learning progresses on from this to find the equation of parallel and perpendicular lines in the form ax + by +c = 0.

This unit takes place in Term 3 of Year 9 and is followed by graphical functions.

Equations of Straight Line Graphs Lessons

Prerequisite Knowledge
  • Recognise and describe linear number sequences, including those involving fractions and decimals, and find the term-to-term rule.
  • Generate and describe linear number sequences

Success Criteria
  • Plot graphs of equations that correspond to straight-line graphs in the coordinate plane;
  • Use the form y = mx + c to identify parallel lines
  • Find the equation of the line through two given points, or through one point with a given gradient
  • Identify and interpret gradients and intercepts of linear functions graphically and algebraically

Key Concepts
  • Gradient is a measure of rate of vertical change divided by horizontal change.
  • Linear Graphs
  • Parallel lines have the same gradient
  • The intercept always has the x value equal zero.

Common Misconceptions
  • Students often confuse linear graphs to have the same notation as statistical graphs.
  • The gradient can be calculated from any two points along the graph. Not necessarily from the origin.
  • A linear function does not have to pass through the origin.
  • It is beneficial to create a table of results when plotting a linear function. The coordinate pairs arise from the x and y values.


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