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.

Work out the gradients of an x first before you relate it back to the graph in question

$Gradient (m) =y/x$

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

$Gradient\ of\ li\n\e\ B=1/-1=-1$

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

Therefore $m_A\timesm_B=-1$