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# C4 implict equation question help please! watch

1. y/x - 2x/y =1 where x>0 y>0

show that dy/dx = y/x
2. (Original post by Tenchu)
y/x - 2x/y =1 where x>0 y>0

show that dy/dx = y/x
Right this looks fairly tricky...

y/x - 2x/y = 1

So: [x(dy/dx) - y]/x2 - [y(2) - 2x(dy/dx)]/y2 = 0

[ use of the quotient rule twice ]

x(dy/dx)/x2 - y/x2 - 2y/y2 + 2x(dy/dx)/y2

=> (dy/dx)[x-1 + 2x/y2] = y/x2 + 2y/y2

=> (dy/dx) = [y/x2 + 2y-1]/[x-1 + 2x/y2]

=> dy/dx = [y(x-2 + 2y-2)] / [x(x-2 + 2y-2)]

=> dy/dx = y/x

I bet someone has beaten me to this one.
3. thanks man, it's all so clear now.
4. (Original post by Tenchu)
thanks man, it's all so clear now.
lol. Not to me it isn't.

The implicit stuff is tough.
5. That was a horrible question! Where did you find it?
6. ye it is a tough question, worth 7 marks, it's from a C4 mock paper published by Delphis (the questions are harder than usual)
7. (Original post by Tenchu)
ye it is a tough question, worth 7 marks, it's from a C4 mock paper published by Delphis (the questions are harder than usual)
Is there anyway that you could send the Delphis papers to me? Know of anywhere they are online etc?

I only have Solomon papers and I am running out fast.
8. I don't have a scanner and I don't think they are available online, sorry mate

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