# Thread: Angle between diagonals of a parallelogram

1. ## Angle between diagonals of a parallelogram

A parallelogram has its sides determined by OA = (5,1) and OB = (-1,4), determine the angle between the two diagonals of this parallelogram.

Assuming I have the diagonal vectors going from bottom left to top right and bottom right to top left, the two vectors I'd end up with would be (4,5) and (-6,3), right?

From there do I use the dot product? I get a different answer than the book, which states it as 77.9

2. Originally Posted by IanCarney
A parallelogram has its sides determined by OA = (5,1) and OB = (-1,4), determine the angle between the two diagonals of this parallelogram.
Assuming I have the diagonal vectors going from bottom left to top right and bottom right to top left, the two vectors I'd end up with would be (4,5) and (-6,3), right?
From there do I use the dot product? I get a different answer than the book, which states it as 77.9
If you use $\displaystyle <4,5>~\&~<6,-3>$ as the diagonals you will get the same answer as the text.

3. Originally Posted by Plato
If you use $\displaystyle <4,5>~\&~<6,-3>$ as the diagonals you will get the same answer as the text.
Oh, I get it. Thanks!

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# angle between diagonal of parallogram is how?

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