Therefore, equation of family C is
x2 + (y – a)2 = a2  or x2 + y2 = 2ay (1)
where, a is an arbitrary constant Differentiating both
sides of equation (1) with respect to x,we get
2
2
dy
x
y dx
 
 = 2
dy
a dx
   Fig 9 3
Fig 9