# Differentail Eq. For circle passing through origin

1. Feb 16, 2006

### Isma

can some 1 help me with it plzzz

2. Feb 16, 2006

### Tom Mattson

Staff Emeritus
can u post the qstn plzzz?

3. Feb 16, 2006

### Isma

question is find the differential eq. for circle that passes through origin.

4. Feb 16, 2006

### Tom Mattson

Staff Emeritus
That can't be all there is to it. There are an infinite number of circles that pass through the origin.

5. Feb 16, 2006

### Isma

exactly....i m v confused....thats all wat the question is for assignment i ve to submit tomorrow
i just kno the answer but dont kno anything else
ans: 2ay''+y'(raise to the power 3)=0

6. Feb 16, 2006

### vaishakh

2ay + y^3 = 0.
2a = -y^2
This represents a circle!!!!

7. Feb 17, 2006

### Isma

yesss!!
thx a lot :)

8. Feb 17, 2006

### Isma

but how will we come to this eq. from start if we dint know the DE?

9. Feb 17, 2006

what is the eq you were given for circle that pass in (0,0) ? is it like this one : $$(x-a)^2 +(y-b)^2 = a^2 +b^2$$

Last edited: Feb 17, 2006
10. Feb 17, 2006

### HallsofIvy

How about writing out the entire problem as it was given?
How can that be an answer when there was no "a" in the original question?

In what sense does that represent a circle?

Did you understand what he meant??

Any circle, that passes through the origin can be written
(x- a)2+ (y- b)2= a2+ b2
(I just noticed that ziad1985 said that!)

Differentiating wrt x, 2(x-a)+ 2(y-b)y'= 0.

Differentiating again, 2+ 2y'2+ 2(y-b)y"= 0.

Now combine those into an equation that does not have either a or b in it.

11. Feb 19, 2006

### Isma

actually....this assignment was given just after 1st lecture on DE in class...so i was v much messed up in mind....nd abt that thx heheheh....it made sense at that moment but not in the next 1:)
i really appreciate u helpin me ...i m gonna solve it like that
thx!