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differentiation stationary points

Find the stationary points of the graph y = 1/x + 1/x^2 + 1/x^3 and determine the nature of each.

I found the derivative which I did -x^-2 - 2x^-3 - 3x^-4 (i know you can write it as a fraction as well) and then I set this equal to 0. I then multiplied by x^4 on both sides and got -x^2 - 2x - 3 = 0 and I made i divided it by -1 to give me nice positive numbers to work with: x^2 + 2x + 3 = 0
However, this gives me no real solutions when solving for x.

Could someone please explain how to do this question? Thanks in advance :smile:
(edited 1 year ago)
Reply 1
Original post by strawberry lover
Find the stationary points of the graph y = 1/x + 1/x^2 + 1/x^3 and determine the nature of each.

I found the derivative which I did -x^-2 - 2x^-3 - 3x^-4 (i know you can write it as a fraction as well) and then I set this equal to 0. I then divided by x^4 on both sides and got -x^2 - 2x - 3 = 0 and I made i divided it by -1 to give me nice positive numbers to work with: x^2 + 2x + 3 = 0
However, this gives me no real solutions when solving for x.

Could someone please explain how to do this question? Thanks in advance :smile:

Can you please post a photo of the original question? That graph doesn't have any stationary points.
It seems like you don't understand how to differentiate properly yet.
To me it looks like you multiplied the constants by the negative of the powers and then added one to the power and then made it negative?

To differentiate a polynomial like that you take each term and multiple the constant by the power and then take away one from the power.
"To differentiate a polynomial like that you take each term and multiple the constant by the power and then take away one from the power" This is what I did. If you differentiated this then what would you get ?
Reply 4
Original post by alanb21
It seems like you don't understand how to differentiate properly yet.
To me it looks like you multiplied the constants by the negative of the powers and then added one to the power and then made it negative?

To differentiate a polynomial like that you take each term and multiple the constant by the power and then take away one from the power.

The differentiation looks fine to me. Did you read their equation correctly?
Original post by Notnek
Can you please post a photo of the original question? That graph doesn't have any stationary points.


The question is exactly the same
Reply 6
Original post by strawberry lover
The question is exactly the same

No it's not. You wrote this:

y = 1/x + 1/x^2 + 1/x^3

But it's actually this:

y = 1/x + 1/x^2 - 1/x^3

Try the question again with this correction.
Original post by Notnek
No it's not. You wrote this:

y = 1/x + 1/x^2 + 1/x^3

But it's actually this:

y = 1/x + 1/x^2 - 1/x^3

Try the question again with this correction.


oh myyyyyyyyyyyyyyy - thank you so much. thank youuuuu !
Original post by Notnek
The differentiation looks fine to me. Did you read their equation correctly?


Oh yeah sorry I got confused because I didn't consider that it's a fraction and just thought that OP was separating the constant with the x term using a slash. My bad!

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