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Equations of lines – WJECParallel lines - Intermediate and Higher tier

Look at how to plot in all four quadrants of a graph and draw straight lines. We can determine information about the gradient and position of the lines, such as if they are parallel or perpendicular.

Part of MathsAlgebra

Parallel lines - Intermediate and Higher tier

Lines which are parallel have the same gradient.

For example, \(\text{y = 2x + 1}\) and \(\text{y = 2x - 2}\) will be parallel because they both have a gradient of 2.

A graph with two solid slanted lines. One line is labelled  'y = 2x + 1' and the other 'y = 2x - 2'.

Example

We want to determine if the lines \(\text{y = 4x + 5}\) and \(\text{2y - 8x = 6}\) are parallel.

The line \(\text{y = 4x + 5}\) is already in the form \(\text{y = mx + c}\) so we know it has a gradient of 4.

We will need to rearrange \(\text{2y - 8x = 6}\) into the same form to see if they are parallel.

\(\text{2y - 8x = 6}\)

Adding \(\text{8x}\) to both sides gives us \(\text{2y = 8x + 6}\).

But we need our equation to start with y on its own so we need to divide all the terms by 2, which gives us \(\text{y = 4x + 3}\).

This is now in the form \(\text{y = mx + c}\), so we can see that it also has a gradient of 4. Therefore, the lines are parallel.

Question

Are the lines \(\text{y = 3x - 1}\) and \(\text{3y - 6x = 9}\) parallel?