Finding the Cosine of the Angle Between Diagonals in a Parallelogram

[Final777]

Hey everyone, I'm pretty new here but a friend referenced this place and I thought I'd ask here. Some simple Linear algebra homework but I can't seem to get my head around this one.

Find the cosine of the angle Phi between the diagonals of a parallelogram whose sides are equal to a and b, if the angle between them is alpha.

I'm aware the helpers here would rather I showed some work on my part prior to asking for help, but I'm rather stumped on this, so any hints/help would be quite welcome. =|

 

[sutupidmath]

Hi Final777, and welcome to Physics Forums!

This is not actually how things go around here, especially in the homework forums. You first must show us what you have tried so far, for only then the people here will try to help you!

 

[HallsofIvy]

To say you are stumped, you must have tried something so show us what you tried. One difficulty with not showing any work is that there are often several different ways of doling a problem and we cannot tell what method would be appropriate for you without seeing what you do know.