Posted by **sally** on Tuesday, April 13, 2010 at 12:08am.

If two vectors u and v fit the equation (u − v) • (u − v) = u•u+v•v, how must these vectors u and v be related? What familiar theorem does this equation represent?

- vector -
**drwls**, Tuesday, April 13, 2010 at 2:44am
(u − v) • (u − v) = u•u + v•v -2u•v

Is what you get by multiplying out the dot products.

For the equation

(u − v) • (u − v) = u•u + v•v

To be valid, u•v must be zero, so u and v must be perpendicular. In that case the triangle formed by u, v and the hypotenuse u -v is a right triangle and the left side is the square of the hypotenuse, as required by the Pythagorian theorem.

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