Posted for Usycool:
When perpendicular lines meet, the product of their gradient is -1, right?
However, say if we have the lines y = 3 and x = 5.
The gradient of the y=3 in undefined, and the gradient of the x = 5 is 1.
So is this not an exception for the rule that perpendicular lines's gradients have a product of -1? Or is their another explanation for this?