Perpendicular lines

`m_A\times\m_B=-1`

Two lines are perpendicular if you multiply the gradients of the two lines together and the
result is -1.

To remember this formula work out what the slopes of an X would be.

`Gradient (m) =y/x`

`Gradient\ of\ li\n\e\ A=1/1=1`

`Gradient\ of\ li\n\e\ B=1/-1=-1` 

This shows when we multiply them together `1\times-1 = -1` 

Therefore `m_A\timesm_B=-1`