Question

Find y" by implicit differentiation
9x^2 + y^2 =9

Answer 1

bye girl dont go posting questions on our chat log

Answer 2

Why can't I post it here?

Answer 3

what is your workings for y'?

Answer 4

\(\Large\bf \color{#CC0033}{\text{Welcome to OpenStudy! :)}}\)

Starting with a circle? Oo these are fun. Oh wait that's not a circle :d

Answer 5

i was joking

Answer 6

I got my y' as (-9x)/y
And y" as (-9y-9xy')/y^2
I was trying to substitute y' back into y"
But then I got (81xy+81x^2)/y^3
However the answer is ((9)(9x^2+y^2))/y^3

Answer 7

9x^2 + y^2 =9

9(2)x + 2y y' =0

9x + y y' =0 ; your y' is fine

now derive again; notice that yy' is a product tho

Answer 8

And I don't understand why is there a y^2 on top

Answer 9

9x + y y' =0

9 + y'y' + yy'' = 0

yy'' = -(9 + y'^2)

y'' = -(9 + y'^2)/y

Answer 10

y is a function of x, to take the derivative of y w.r.t. x, you have to use the chain rule

Answer 11

y'' = -(9 + y'^2)/y

y'' = -(9 + (-9x/y )^2)/y

y'' = -(9 + 81x^2/y^2)/y

Answer 12

multiply top and bottom by y^2 to clear the fraction in the numerator

Answer 13

\[\frac{ d }{ dx }y(x) = \frac{ d }{ dy }y(x)*\frac{ dy }{ dx }\]

Answer 14

`I got my y' as (-9x)/y`
`And y" as (-9y-9xy')/y^2`
These steps look correct ^
Maybe you just ran into a little trouble plugging in your y'.
Hmm.

Answer 15

Oh there should be another negative in the numerator, I missed that.
-(-9x)y'

Answer 16

It looks like you chose the quotient rule route, so remember that it's subtraction in the numerator, and we're carrying around a (-9x).

Answer 17

As my y' is (-9x/y)
I used quotient rule to derive my y"
That's y I came up with :
Y"=(-9y-9xy')/y^2
y"= (-9y-9x)(-9x/y) / y^2
Y"= (81xy+81x^2)/y^3
However the answer shows:
9(9x^2+y^2)/y^3

Answer 18

using product (it tends to be simpler to follow)

y' = -9xy^(-1)

y'' = -9y^(-1) + 9xy'y^(-2)

-9y^(-1) + 9x(-9xy^(-1))y^(-2)

-9 y^(-1) -81x^2 y^(-3)

Answer 19

you sure the answer is without a negative?

Answer 20

There is a negative sign I'm sorry I missed it

Answer 21

\[\large\rm y''=\frac{-9y-(-9x)\color{orangered}{y'}}{y^2}=y''=\frac{-9y-(-9x)\color{orangered}{\frac{(-9x)}{y}}}{y^2}\]Depending on how you proceeded from here....
I would probably choose to factor a 1/y from each term in the numerator,\[\large\rm y''=\frac{1}{y}\cdot\frac{-9y^2+81x^2}{y^2}\]That last step is one giving you trouble? :o

Answer 22

I missed a negative again :) lol\[\large\rm y''=\frac{1}{y}\cdot\frac{-9y^2-81x^2}{y^2}\]

Answer 23

Ok I'm trying to figure it out right now. Thanks for all of your help:)!!!!!!!!!!!! Thanks!