Solving for the Angle Between Vectors A and B

[LVanderlinden]

1. Homework Statement

Vectors A and B have equal magnitudes of 15.0. If the sum of A and B is the vector 4.55 j, determine the angle between A and B.


3. The Attempt at a Solution

I thought I know what I was doing but apparently not. Please help!

 

[Oxvillian]

Try looking at

$$({\bf A} + {\bf B})\bullet({\bf A} + {\bf B})$$

 

[Simon Bridge]

It helps if you show how you have attempted it.

Have you tried sketching two equal magnitude vectors with some arbitrary angle between them, then working out the equation for the magnitude of the sum as a function of the angle? (there is a simplification if you put the y-axis along the resultant vector.)