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# Finding x value knowing the gradient from f'(x) Watch

1. I managed to do part a) without much issue, but part b) I did with by substitution, which was not the method on the mark scheme and apparently, not the right answer. Where did I make the mistake in my workings?

2. I think you put x=3 into f(x) so f(3)=....

Don't no exactly until I do the whole question. If that not right let me no I will do the question for you
3. (Original post by frostyy)
...
You were doing perfectly fine right up till .

To get this into a quadratic, you need to multiply both sides by which gets you which gives you your values of . I'm not sure what you seem to have done.
4. (Original post by Aaliyah.97)
I think you put x=3 into f(x) so f(3)=....

Don't no exactly until I do the whole question. If that not right let me no I will do the question for you
It's not right. You don't know what is. Also don't bother doing the question for him, full solutions are against forum guidelines.
5. (Original post by Zacken)
It's not right. You don't know what is. Also don't bother doing the question for him, full solutions are against forum guidelines.
Ooh okay I didn't know that I'm knew here so sorry
6. Don't you just do the original equation they gave you (as it is the differentiated equation) equal to 10 and the two values of x you find is your answer
7. (Original post by Aaliyah.97)
Ooh okay I didn't know that I'm knew here so sorry
No problem! :-)
8. (Original post by Student232)
Don't you just do the original equation they gave you (as it is the differentiated equation) equal to 10 and the two values of x you find is your answer
Yes. That's precisely what he's done, except for a small slip when trying to solve his quadratic in .
9. (Original post by Zacken)
You were doing perfectly fine right up till .

To get this into a quadratic, you need to multiply both sides by which gets you which gives you your values of . I'm not sure what you seem to have done.
Thanks so much! I tried to get rid of the (9/t) by multiplying both sides by 9. Not sure why I thought of that, but it didn't occur to me that that was were the issue was lying. Anyway, thanks a lot
10. (Original post by frostyy)
Thanks so much! I tried to get rid of the (9/t) by multiplying both sides by 9. Not sure why I thought of that, but it didn't occur to me that that was were the issue was lying. Anyway, thanks a lot
No problem!

You might have been able to avoid this issue by having done straight away; that is:

Then the substitution gives you

But your method was essentially the same and just as nice, really!
11. (Original post by Zacken)
Yes. That's precisely what he's done, except for a small slip when trying to solve his quadratic in .
Sorry I didn't look at the working haha only the question
12. (Original post by Student232)
Sorry I didn't look at the working haha only the question
Yeah, don't worry, I'm guilty of doing that far too often.

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