If in a $$\Delta$$ABC, 2b² = a² + c², then $$\frac{\sin 3B}{\sin B}$$ is equal to

The height of the chimney when it is found that on walking towards it 50 m in the horizontal line through its base, the angle of elevation of its top changes from 30$$^\circ$$ to 60$$^\circ$$ is :

A bird is sitting on the top of a vertical pole 20 m high and its elevation from a point O on the ground is 45$$^\circ$$. If flies off horizontally straight way from the point O. After one second, the elevation of the bird from O is reduced to 30$$^\circ$$, then the speed (in m/s) of the bird is