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# C3 Differentiation watch

1. I worked out this question just fine all the way through until the last part, when subbing in the value x = 4, why do they disregard the negative solution of root 9? Would I be wrong in giving the answer +/- 13 instead of just positive 13 for this question? I can't think of any way you would just 'know' to not use -3 as the root of 9.

2. If you had to work it out at the point (4, 36) then there is no need to sub in the other value, but I can't give a proper answer as I don't know what the question is asking you to do. What is the question?

This was a stupid post, I didn't read the question properly.
3. not
4. (Original post by BabyMaths)
not
Since when? I'm really confused now.
5. (Original post by claret_n_blue)
If you had to work it out at the point (4, 36) then there is no need to sub in the other value, but I can't give a proper answer as I don't know what the question is asking you to do. What is the question?
The question and a large part of the solution are attached to the original post.
6. (Original post by Ronove)
Since when? I'm really confused now.
Can't blame you there. Plenty of teachers seem to think that . It doesn't.
7. (Original post by BabyMaths)
not
Since you said this (which shook my understanding to the core by the way ) I found this page http://gmatclub.com/forum/square-roo...ve-114114.html which has blown my mind. Thanks for the response, haha!
8. (Original post by BabyMaths)
Can't blame you there. Plenty of teachers seem to think that . It doesn't.
Well I am my own teacher, haven't had a maths teacher since doing my GCSEs in 2004! I have no idea how I would ever have found this out without you saying it.
9. (Original post by Ronove)
Well I am my own teacher, haven't had a maths teacher since doing my GCSEs in 2004! I have no idea how I would ever have found this out without you saying it.
To clarify if you are asked to solve you write down .

10. (Original post by BabyMaths)
Can't blame you there. Plenty of teachers seem to think that . It doesn't.
Woah! What?
11. (Original post by Mr M)
To clarify if you are asked to solve you write down .

Thank you!
12. (Original post by claret_n_blue)
Woah! What?
What?
13. (Original post by TenOfThem)
What?
Why is it just 2?

Or is this just for exams, in real life it still plus or minus 2 isn't it?
14. Couple of long threads on the maths teachers forum at TES explain why students aren't clear on this.

http://community.tes.co.uk/forums/p/...4.aspx#7840374

http://community.tes.co.uk/forums/p/...3.aspx#7132713
15. (Original post by claret_n_blue)
Why is it just 2?

Or is this just for exams, in real life it still plus or minus 2 isn't it?
No

It never has been

-2 squared does still =4

But the square root of 4 is 2
16. (Original post by claret_n_blue)
Why is it just 2?

Or is this just for exams, in real life it still plus or minus 2 isn't it?

17. (Original post by claret_n_blue)
Why is it just 2?

Or is this just for exams, in real life it still plus or minus 2 isn't it?
No. The square root of x is defined to be the POSITIVE number y which satisfies y^2=x
18. (Original post by TenOfThem)
-2 squared does still =4

But the square root of 4 is 2
Why? How can you not go back the other way?

(Original post by james22)
No. The square root of x is defined to be the POSITIVE number y which satisfies y^2=x
No, I thought the definition was the number which when multiplied by itself gives your original number. This doesn't matter if its positive or negative as (-y)*(-y) = y * y
19. (Original post by claret_n_blue)
No, I thought the definition was the number which when multiplied by itself gives your original number. This doesn't matter if its positive or negative as (-y)*(-y) = y * y
You thought wrong then.
20. (Original post by Mr M)
You thought wrong then.
http://whatis.techtarget.com/definit...re-root-symbol

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