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Sorry if this is a little basic, but my math is pretty rusty. I know the point coordinates of the two blue dots. I also know the distances from the middle blue dot to the edges of the square. How can I calculate the coordinates of the red dot?

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up vote 1 down vote accepted

The two right angled triangles in the picture are similar. Hence the ratio of the corresponding sides are the same. You can use this to get the remaining coordinates.

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