To convert, use #y=r sin theta# and #x=r cos theta#
replace all #x# and #y# the equivalents
#y=y/x-(x+y)^2#
#r sin theta=(r sin theta)/(r cos theta)-( r cos theta+ r sin theta)^2#
#r sin theta=(cancelr sin theta)/(cancelr cos theta)-( r^2)( cos theta+ sin theta)^2#
#r sin theta=(sin theta)/( cos theta)-( r^2)( cos^2 theta+ sin^2 theta+2 sin theta cos theta)#
#r sin theta= tan theta-r^2(1+sin 2theta)#
#r^2(1+sin 2theta)+r sin theta-tan theta=0#