What do you know about the gradient of a perpendicular line?

If we know the gradient of a particular line, for example line L1 with gradient 5, we can calculate the gradient of the line that would intersect it at a right angle (perpendicular). We will call this line L2.

The product of the gradients of L1 and L2 will always be -1. This can also be called the negative reciprocal.

The reciprocal of L1 gradient is 1/5 and so the negative reciprocal will be -1/5. This is the gradient of the perpendicular line L2