Posted by Hannah on Sunday, March 28, 2010 at 5:08pm.
\vec b\cdot \vec c = \Vert \vec b\Vert\Vert\vec c\Vert\cos \theta
The law of cosines in vector form:
b dot c= abs b * abs c *CosA
(3i+j)dot(2i-j)=sqrt(10)*sqrt(5) CosA
CosA=5/5sqrt2= .707
solve for A