find a.b if a=|10| and b=|2| , and the angle between a and b is -pi/3 radians.

Question

find a.b if a=|10| and b=|2| , and the angle between a and b is -pi/3 radians.

Answer 1

The formula for the cosine(angle between two vectors) is 

 

cosΘ=(a•b)/[|a|*|b|]

 

So plug in what you know...

 

cos(-pi/3) = a•b / [(10)(2)]

 

so 

a•b=(10)(2)cos(-pi/3) = 20(0.5) = 10