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Chapter 13: Straight Line graphs

Line graphs

  • Here are two things you need to know about straight line graphs
  • If an equation is in the form y =mx + c, its graph will be a straight line
  • Use a table of values to draw a graph

Finding equations

  • If you have a graph you can found its equation by working out the gradient and looking at the y-intercept
  • Put your values for gradient, m, and y -intercept,c, into the equation of a straight line y=mx + c

Working in out

  • You can rearrange the equation of this graph into the form y=mx+c so it is a straight line
  • The gradient is a and the y-intercept is at()
  • You could use this information to draw the graph, but its safer to make a table of values
  • Make sure you plot at least three points, then join them with a straight line using a ruler

Methods

  • Given one point and the gradient
    • Substitute the gradient for m in y=mx + c
    • Substitute the x-values and y-values given into the equation
    • Solve the equation to find c
    • Write out the equation
  • Given two points
    • Draw a sketch showing the two points
    • Work out the gradient of the line using a triangle
    • Use method 1 and one of the points given to find the equation

Positive or negative

  • If the line slops down then the gradient is negative

Examiners report

  • Make sure you know how to find the equation of a line through two points
    • Draw a sketch of the line
    • Use a triangle to find the gradient
    • Substitute the gradient and the x and y-values of one of the points into y=mx+c
    • Solve your equation to find c