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