SOLUTION: If sum of three vectors A, B and C is zero and Modulus of A is 3, modulus of B is 5 and modulus of C is 7. Find angle between vectors A and B?

The given information about the vectors MEANS THAT three vectors lie in one plane

and form a TRIANGLE  ABC  in this plane.


Of this triangle, we are given the side lengths, and the problem asks to find the angle between the sides of the length 3 and 5,
while the opposite side is of 7 units long.


So, we write the Cosine Law formula


     = 7


then simplify it and find the angle    between the sides A and B


     = 

     = 49

    9 + 25 - 49 = 

     = -15

     = 

     = 

     = 120° = .      ANSWER


ANSWER.  Under the given condition, the angle between the vectors A and B is 120°,  or   radians.