Parallel lines have the same gradient.
All of these lines have a gradient of 5.
Perpendicular lines have gradients which multiply together to give -1.
Here one line has a gradient of 5 and the other of -1/5.
To find the gradient of a line which is perpendicular to one of gradient a/b, we invert the fraction and multiply by -1.
This means you don't need diagrams to check if lines are perpendicular.
State the gradient of a line which is perpendicular to one of gradient ¾.
Invert the fraction and multiply by -1.