Thread: 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

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 as the diagonals you will get the same answer as the text.

Originally Posted by Plato

If you use as the diagonals you will get the same answer as the text.

Oh, I get it. Thanks!