The equation of family of circles of fixed radius' r′ with centres on the y -axis is (x−0)2+(y−a)2=r2.....(i) On differentiating w.r.t. x, we get 2x+2(y−a)‌
dy
dx
=0 ⇒‌‌‌
dy
dx
=−‌
x
y−a
⇒‌‌(y−a)=‌
−x
(dy/dx)
On putting this value in Eq. (i), we get x2+‌
x2
(‌
dy
dx
)2
=r2 ⇒‌‌x2{1+(‌
dy
dx
)2}=r2(‌
dy
dx
)2 Hence, order → highest order derivative =1 and degree → power of highest order derivative =2