5. We shall now apply this method to a few typical cases.
(i) The equation y 1=x 2+y 2
gives y 2=2x+2y 2(x 2+y 2).
Hence, y 1 is always positive; y 2 vanishes along the curve
r 2=-cot θ,
in which x=r cos θ, y = r sin θ. Hence, in Fig. 5(a), the curves have the form) above the dashed curve (the locus of inflexion), and ( below this curve.