The differential equation of the family of circles passing through the origin and having centre at the line y = x is :
- (x2 - y2 + 2xy )dx = (x2 - y2 + 2xy )dy
- (x2 + y2 - 2xy )dx = (x2 + y2 + 2xy )dy
- (x2 + y2 + 2xy )dx = (x2 + y2 - 2xy )dy
- (x2 - y2 + 2xy )dx = (x2 - y2 - 2xy )dy
Answer
(4) (x2 - y2 + 2xy )dx = (x2 - y2 - 2xy )dy
Solution
Centre (h, h), passes through O r2 = 2h2: x2 + y2 = 2h(x + y) ...(i)
Differentiate: 2x + 2yy' = 2h(1 + y') 2h = 2(x + yy')1 + y'
Put in (i): (x2 + y2 )(1 + y') = 2(x + y)(x + yy')
y' (x2 + y2 - 2xy - 2y2 ) = 2x2 + 2xy - x2 - y2
y' (x2 - y2 - 2xy ) = x2 - y2 + 2xy
(x2 - y2 + 2xy )dx = (x2 - y2 - 2xy )dy
Answer: (4)