# How can the law of cosines be used to find the magnitude of a resultant?

If we are to add vector A and B and if $A$ and $B$ be their magnitudes, then
If R is the resultant and $R$ it's magnitude,
then $R = {\left({A}^{2} + {B}^{2} + 2 A B C o s \theta\right)}^{\frac{1}{2}}$
where $\theta$ is the angle between A and B when they are drawn tail to tail.