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The Diagram

The blue dot moves, I know its x and y coordinates, and I also know that that causes the red dot to move along the triangle's hypotenuse in such a way that the red dot's y coordinate is always equal to the blue dot's y-coordinate.

How do I calculate the x value of the red dot's coordinate?

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    If the red dot starts at the bottom left vertex and the origin is there, $y_{\text{red}}/x_{\text{red}}$ is the slope of the hypotenuse. Modifications can be made if the red dot starts somewhere else and you know the initial coordinates.2011-12-11
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    Thanks @david-mitra. But how can I calculate the x values of the red dot's coordinate if I know the x and y values of the blue dot (and the slope of the hypotenuse)?2011-12-11
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    That's still not quite enough information. You also need to know where the triangle is on the plane.2011-12-11
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    @DustanLevenstein: Ok, well let's assume the leftmost vertex of the triangle is at 0,0.2011-12-11
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    http://en.wikipedia.org/wiki/Dot_product#Geometric_interpretation2011-12-12

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