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C4 Implicit functions

Hi can someone show me where i am going wrong with these?

Find the gradient of the stated curve at the specified point.

2C) 3y47xy212y=5 3y^4 -7xy^2-12y = 5 at the point (2,1) (-2,1)

I differentiate to get:

12y3dydx(14xydydx+7y2)12dydx=0 12y^3\dfrac{dy}{dx} - (14xy \dfrac{dy}{dx} +7y^2) - 12 \dfrac{dy}{dx} = 0

I get dydx \dfrac{dy}{dx} on its own.

dydx=7y212y314xy12 \dfrac{dy}{dx} = \dfrac{7y^2}{12y^3-14xy -12}

I sub the values in and get 728 \dfrac{7}{28}, however the answer in my book says it is 740 \dfrac{7}{40} .
Reply 1
Nvm yh I got same as you.
(edited 11 years ago)
Book's wrong.
Reply 3
I won't lie I get the same as you. Are you sure that you have the right equation and point?
Reply 4
I agree with your answer; it is 7/28= 1/4.

Book is wrong.
Reply 5
Original post by 0range
I think it should be -7y^2



What?

:confused:
Reply 6
Original post by f1mad
What?

:confused:


lol nah its nothing i forgot to put a minus when doing the product rule on the -7xy^2
Reply 7
Original post by 0range
Nvm yh I got same as you.



Original post by ghostwalker
Book's wrong.



Original post by miss katie
I won't lie I get the same as you. Are you sure that you have the right equation and point?


Yeah, i doubled checked the question, i've spent over an hour on this question :s-smilie:.
Original post by f1mad
I agree with your answer; it is 7/28= 1/4.

Book is wrong.


Ok thanks guys :smile:

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