and from diagram we can say that total rotation is 2θ then the restoring moment 
iii) Taking Moment equilibrium at point B
From above equation we will get one trivial solution 
and second solution gives us the critical load
At the critical value of the load the structure is in equilibrium regardless of the magnitude of the angle 
If 
the structure is stable
If 
the structure is unstable
If 
the structure is neutral equilibrium
