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# C4 implict differentiation question watch

1. hi , for this question I am stuck on the y values I obtained. From my working , there are two y values worked out for each solution of x. But the mark scheme only has on y value for each x value.Attachment 618352618356

ThanksAttachment 618352618356
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2. (Original post by coconut64)
hi , for this question I am stuck on the y values I obtained. From my working , there are two y values worked out for each solution of x. But the mark scheme only has on y value for each x value.Attachment 618352618356

ThanksAttachment 618352618356
Mark scheme attachment doesn't seem to be working, but you're supposed to substitute x = 2, -2 (assuming this is correct) back into the y = -x equation, not into the original curve equation.
3. (Original post by tajtsracc)
Mark scheme attachment doesn't seem to be working, but you're supposed to substitute x = 2, -2 (assuming this is correct) back into the y = -x equation, not into the original curve equation.
why is that?
4. (Original post by coconut64)
why is that?
When , you have found that so you need to find where on the curve. To do that you need to solve these simultaneous equations:

You solved these equations to get and . If you remember (GCSE) you can normally plug in these into either of your simultaneous equations to get the values. So it's best to choose the simpler one . You can use the more complicated equation but that will give you two solutions and only one of them will also be valid in so you'd have to check.

So it makes sense to always plug the values into the simpler equation. Does this make sense?

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