# Angle between diagonals of a parallelogram

• May 26th 2011, 01:23 PM
IanCarney
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
• May 26th 2011, 01:35 PM
Plato
Quote:

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.
• May 26th 2011, 01:37 PM
IanCarney
Quote:

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!