$ABC$ is an equilateral triangle with $O$ as its centre. $\vec{F_1},\vec{F_2}$ and $\vec{F_3}$ represent three forces acting along the sides $AB, BC$ and $AC$ respectively. If the total torque about O is zero then the magnitude of $\vec{F_3}$ is
Taking anticlockwise torque \(F _{1} d + F _{2} d - F _{3} d =0\) (where \(d =\) perpendicular distance of the centre \(O\) from each side) \(\therefore F _{3}= F _{1}+ F _{2}\)