Finding the Angle Between Vectors A & B

[Rail24]

Homework Statement

Vectors A and B have equal magnitudes of 5.12. If the sum of A and B is the vector 6.37j, determine the angle between A and B.

Homework Equations

Cosine law?

The Attempt at a Solution

I attempted to do this using the cosine law which I thought made sense but 103 degrees is not the right answer..

 

[indietro]

when i use the cosine equation:
cosA = (b^2 +c^2 - a^2)/2bc
A = cos-1[(b^2 +c^2 - a^2)/2bc]
i did not get 103 degrees... id suggest to check your numbers??

 

[Rail24]

You got 77 degrees correct?
I just subtracted from 180 because the angle I assume is tail to tail.

I also tried 77 degrees (or rather 76.9 degrees) and it's also not the answer.