Two forces of equal magnitude F act at an angle of 120 degrees to each other. Their resultant has magnitude

Explanation

Using the parallelogram rule, the resultant is the square root of (F squared plus F squared plus 2F squared cos 120), and since cos 120 is minus one half the last term cancels one of the others, leaving the square root of F squared, which is F. Equal vectors at 120 degrees are the arrangement in which three such vectors would sum to zero. At 0 degrees the resultant would be 2F and at 180 degrees it would be zero.