s=√(x^2+y^2)ds/dt=(1/2)[1/√(x^2+y^2)] . d/dt (x^2+y^2)=(1/2)[1/√(x^2+y^2)] . ( 2x . dx/dt + 2y. dy/dt)=[1/√(x^2+y^2)] . ( x (30)+ y. (80/3)x )
