In the applet below, lines f and g are parallel lines. [br][br]Interact with this 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][math]l_1[/math] is [b]parallel [/b]to [math]l_2[/math] [math]\Longleftrightarrow[/math] [math]m_1=m_2[/math][/center]

If three points are on the same line, they are called [b]collinear [/b]points.[br][br]Try to make a collinear scenario out of A, B, and C in the above applet.[br][br]You will notice that the gradient of AB = gradient of AC = gradient of BC, or [math]m_{AB}=m_{AC}=m_{BC}[/math]