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