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# C1 Differentiation watch

1. Hi, how do I do question 8iii?

http://i.imgur.com/iomYmdR.png

I differentiated the equation to 9x^2 - 2x^-2 - 7 = 0

Then I used the logic that x = 0 and y = -7, yet the mark scheme said that the answer was y = -2.

Thank you.
2. (Original post by Peppercrunch)
Hi, how do I do question 8iii?

I differentiated the equation to 9x^2 - 2x^-2 - 7 = 0

Then I used the logic that x = 0 and y = -7, yet the mark scheme said that the answer was y = -2.

Thank you.
We can't see the question, can you post an image/link?
3. Question:
4. (Original post by Peppercrunch)
Question:
You cannot substitute x = 0 into the differential to find the y value, it is true that x = 0 at Q but you need to find the equation of the tangent.
5. (Original post by lizard54142)
You cannot substitute x = 0 into the differential to find the y value, it is true that x = 0 at Q but you need to find the equation of the tangent.
How would I find the equation of the tangent?
6. (Original post by Peppercrunch)
How would I find the equation of the tangent?
Sub x=1 into the original equation of the curve. As the tangent at the stationary point will have a gradient of 0, it will just be a line, and so the value when the tangent crosses the x axis will be whatever you get when you sub x=1 into the original equation of the curve.
7. (Original post by aoxa)
Sub x=1 into the original equation of the curve. As the tangent at the stationary point will have a gradient of 0, it will just be a line, and so the value when the tangent crosses the x axis will be whatever you get when you sub x=1 into the original equation of the curve.
Oh, thank you. Didn't think it would be that easy.
8. (Original post by Peppercrunch)
Oh, thank you. Didn't think it would be that easy.
You're welcome - if you're not sure how hard it should be, just look at the number of marks, if you're doing loads of working for a 1/2 mark question, you're probably missing something obvious

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