When we convert polar coordinates(r,theta) to Cartesian coordinates, the relation is x=rcostheta, y=rsintheta and hence r^2=x^2+y^2 and theta=tan^(-1)(x/y).
Hence (r+1)^2=theta+sectheta can be written as
r^2+2r+1=theta+sectheta or
x^2+y^2+2sqrt(x^2+y^2)+1=tan(-1)(x/y)+sqrt(x^2+y^2)/x or