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# P4 exercise 5c question 2 watch

1. does n e one hav a clue how to do this??? i start off dividing everythin by 2xy, but then u've got an xy term on the other side of the equals sign which i can't get rid of. no surprises the book doesn't giv u an example which u can folow to this question....

n e help wud b great

Dake
2. I don't have the book. Could you post the question?
3. yea sure its a first order diff one....

find the general solution of

2xy dy/dx + y^2 = x^3
4. (Original post by DaKe)
does n e one hav a clue how to do this??? i start off dividing everythin by 2xy, but then u've got an xy term on the other side of the equals sign which i can't get rid of. no surprises the book doesn't giv u an example which u can folow to this question....

n e help wud b great

Dake
I think you have to spot that the LHS is the exact derivative of xy²

Aitch
5. This something you wiil spot with (lots of) practice ...

2xy dy/dx + y² = d/dx (xy²)

etc. ...
6. (Original post by Aitch)
I think you have to spot that the LHS is the exact derivative of xy²

Aitch

Note that this section of the exercise is prefaced with "In Q 1-5 the differential equations are exact." So the LHS is the exact derivative of a product.

Aitch
7. thanks for ur help guys

yep ur both correct but how can i practice these??? and wher did u learn to notice them? is ther som kinda special trik i don't know about???
8. (Original post by DaKe)
thanks for ur help guys

yep ur both correct but how can i practice these??? and wher did u learn to notice them? is ther som kinda special trik i don't know about???
The fact that this section of Ex. 5c gives you a big clue tells you (I hope) that the big clue is necessary here. I'm doing P4 on Tuesday, and would hope not to be presented with an exact FOD without the clue!

Aitch
9. (Original post by DaKe)
thanks for ur help guys

yep ur both correct but how can i practice these??? and wher did u learn to notice them? is ther som kinda special trik i don't know about???
Do lots of DE questions, and study the answers/solutions.
Search these forums for other threads on DE's.
See what they are like.

Best of luck.
10. (Original post by Aitch)
The fact that this section of Ex. 5c gives you a big clue tells you (I hope) that the big clue is necessary here. I'm doing P4 on Tuesday, and would hope not to be presented with an exact FOD without the clue!

Aitch

I've yet to see an exact FOD set on a past paper, or a Solomon paper.

It occurred to me just after logging off yesterday that a fair way to set one would be as a "Show that..." so that you could differentiate the target to get back to the integral.

Aitch

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