Click and drag the points A and D to define the line.[br]The direction of the line is controlled by the direction vector [b]d[/b] (using point D).[br]The line passes through the point A in a direction defined by the vector [math]\lambda \mathbf{d}[/math] where [math]\lambda[/math] is a parameter which can be varied with the slider.[br]The vector equation of the line is a parametric equation of the form [math]\mathbf{r}=\mathbf{a}+\lambda \mathbf{d}[/math].