Allie, Beth, and Cathy are having a 3 way tug of war. Allie is pulling with a force of 30N, Beth with 50N, and Cathy with 70N. If the system is in equilibrium, what is the angle between Allie's rope and Beth's rope.

I used the cosine law, and

$\displaystyle cos\theta=\frac{{30}^2+{50}^2-{70}^2}{2\cdot 30\cdot 50}$

$\displaystyle \theta = 120$

But the answer is 60, because 180-120=60. Why?