Applet: A point on the line determined by two vectors
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The point $\color{blue}{\vc{x}}$ (represented by the blue point as well as the blue vector) is constrained to be on the line passing through the point $\color{red}{\vc{a}}$ (represented by the red vector) and parallel to the vector $\color{green}{\vc{v}}$ (in green). You can move $\color{blue}{\vc{x}}$ by dragging it with the mouse, but it is constrained to be on the line. You can change the line by dragging $\color{red}{\vc{a}}$ or $\color{green}{\vc{v}}$ with the mouse.
Since $\color{blue}{\vc{x}}$ is on the line, the vector $\color{green}{\vc{v}}$ is always parallel to the vector $\color{cyan}{\vc{x}-\vc{a}}$ (in cyan).
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General information about three.js applets
The applet was made using three.js and requires Javascript as well as a browser that supports WebGL. For most three.js applets, you can drag with the mouse to rotate the view, drag with the right button to pan, and zoom in/out with the mouse wheel. Many applets contain points that you can drag to change values of variables.