1. The problem statement, all variables and given/known data Vectors A and B have the same magnitude. |A+B| is 78 times greater than |A-B|, for this to happen, what must be the angle between vectors A and B? 2. Relevant equations 78((A^2) + (B^2)-2ABcos(theta))=((A^2)+(B^2)+2ABcos(theta)) Made it by using dot product. 3. The attempt at a solution What I did was used dot product and got 78((A^2) + (B^2)-2ABcos(theta))=((A^2)+(B^2)+2ABcos(theta)). And I let B=A because they have the same magnitude. I simplify this equation further and cancel things out and I got the wrong answer. The answer I got is 12.9 deg. What did I do wrong?