In the applet below, lines [math]l_1[/math] and [math]l_2[/math] are perpendicular lines.[br][br]Interact with the applet by dragging points A, B, and C.
In general, for two lines[math]l_1[/math] and[math]l_2[/math] with gradients [math]m_1[/math] and [math]m_2[/math] respectively,[br][br][center][size=150][math]l_1\perp l_2\Longleftrightarrow m_1\times m_2=-1[/math][/size][/center]