What should be the angle between two vectors of magnitudes 3.20 and 5.70 units, so that their resultant has a magnitude of 6.10 units?

Answer 1

Use the law of cosines.
c^2 = a^2 + b^2 - 2ab cos C
6.1^2 = 3.2^2 + 5.7^2 - 2*3.2*5.7 cos C
cos C = (10.24 + 32.49 - 37.21)/36.84
= 0.1498
Solve for C and verify my numbers.