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# C4 help watch

1. Q14)

I found dy/dx = (3y-3x^2)/(3y^2-3x)

What do I from here?
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2. Okay I equated dy/dx=0

And got y=x^2
3. (Original post by Vorsah)
Okay I equated dy/dx=0

And got y=x^2
See what happens when you substitute that equality into the original expression in terms of x and y.
4. (Original post by Khallil)
See what happens when you substitute that equality into the original expression in terms of x and y.
I get x=3^1/3
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5. (Original post by Vorsah)
I get x=3^1/3
Try again.
6. (Original post by Khallil)
Try again.
Divided through by x^3
7. (Original post by Vorsah)
Divided through by x^3
First of all, you shouldn't be dividing by as you don't know if could equal 0.

Second of all, what's the value of
8. (Original post by khallil)
try again.
x^3 + x^6 - 2x^3=0

x^6 - 2x^3=0

x^3(x^3 - 2)=0

x=0
x=2^1/3?
9. (Original post by Vorsah)
x^3 + x^6 - 2x^3=0

x^6 - 2x^3=0

x^3(x^3 - 2)=0

x=0
x=2^1/3?
My working concurs with yours. Now find their respective y coordinates.

Do you know how to verify whether a point is a maximum, minimum or point of inflection?
10. (Original post by Khallil)
My working concurs with yours. Now find their respective y coordinates.

Do you know how to verify whether a point is a maximum, minimum or point of inflection?
Yep
11. (Original post by Vorsah)
Yep
That's good to hear

Out of interest, what are the answers at the back of the book for this question?
12. Max at 2^2/3
Min at 0
13. (Original post by Khallil)
That's good to hear

Out of interest, what are the answers at the back of the book for this question?
I got d^2y/dx^2 = -6x/(3y^2-3x)

When sub in (2^1/3, 2^2/3) I get -2 = max, which is correct

When I sub in (0,0) , I can't do it because you do 0/0, so how am I suppose find out?
14. (Original post by Vorsah)
I got d^2y/dx^2 = -6x/(3y^2-3x)

When sub in (2^1/3, 2^2/3) I get -2 = max, which is correct

When I sub in (0,0) , I can't do it because you do 0/0, so how am I suppose find out?
I also got 0/0 for my second derivative evaluated at the point (0, 0) and I'm unsure of how to resolve that.
15. Any1 else?

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Updated: November 16, 2013
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