The length of the common chord of the circles $$x^2+y^2+3x+5y+4=0$$ and $$x^2+y^2+5x+3y+4=0$$ is __________ units.

Find the equation of the circle which passes through the point $$(1,2)$$ and the points of intersection of the circles $$x^2+y^2-8 x-6 y+21=0$$ and $$x^2+y^2-2 x-15=0$$

Given, two fixed points $$A(-2,1)$$ and $$B(3,0)$$. Find the locus of a point $$P$$ which moves such that the angle $$\angle A P B$$ is always a right angle.

The equations of the tangents to the circle $$x^2+y^2=4$$ drawn from the point $$(4,0)$$ are