# How do you find the angel between u=5i-4j, v=2i+j?

u.v=∥u∥∥v∥cos(u,v)
cos(u,v)=(u.v)/(∥u∥∥v∥)
$u . v = 5 \cdot 2 - 4 \cdot 1 = 6$
∥u∥=sqrt(5^2+4^2)=sqrt41
∥v∥=sqrt(2^2+1^2)=sqrt5
$\cos \left(u , v\right) = \frac{6}{\sqrt{41.} \sqrt{5}} = 0.6266$
$\left(u , v\right) = 0.894 r d$