If two forces of 5 lb. and 8 lb. are acting on the same point are equivalent to a single force of 10 lb., find the angle between the given forces, and the angle between the resultant and the larger force.

Answer 1

use the law of cosines

10^2 = 5^2+8^2 - 2*5*8*cos(180-θ)

Answer 2

I'm not allowed to. I have to use vector methods of solving this.