Dot Product from the magnitudes of the vectors and the angle between them:
Find
,
where |a| = 5 and |b| = 2 and the angle q between them is 150 degrees.
In this case the Dot Product is the product of the magnitudes and the cosine of the angle between them:
s = |a| * |b| * cos q . Here this becomes
s = 5 * 2 * cos(150) = 10 * (-.866) = -8.66.