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# C3 Question- BAM! watch

1. Really stuck on c3 question. any help appreciated:

given that f(x) = cotx + 3

solve f(x) + f '(x) = 0 for 0 < x < 2pie, to 1 d.p

rep on offer
2. A good place to start might be to calculate f'(x)...
3. (Original post by Kolya)
A good place to start might be to calculate f'(x)...
-cosec^2 x. but then i get stuck...
4. (Original post by Kolya)
A good place to start might be to calculate f'(x)...
if you could help me you would save my life. if i get this qu. done i can go to bed lol
5. If I tell you that 1 + cot^2(x) = cosec^2(x) that should help
6. well and

plus as said above

re arrange it from there and you should be on the right track
7. Actually no, all you need to know is what DLS said! I gave you too much info!
8. (Original post by DLS)
If I tell you that 1 + cot^2(x) = cosec^2(x) that should help
ahhh cheers . i knew i was missing something. i am now left with a quadratic which factorises. Bed here i come woooo
9. (Original post by Tommy Jay)
Really stuck on c3 question. any help appreciated:

given that f(x) = cotx + 3

solve f(x) + f '(x) = 0 for 0 < x < 2pie, to 1 d.p

rep on offer
Cotx +3 -Cot^2 x -1 =0
Cot^2 x - Cot x -2 = 0
(C-2)(C+1)
cotx = 2
cotx=-1
tanx=(1/2)
tanx=(-1)
etc.
11. (Original post by Tommy Jay)
ahhh cheers . i knew i was missing something. i am now left with a quadratic which factorises. Bed here i come woooo
LOL.
Good luck!
12. (Original post by n_251)
well and

plus as said above

re arrange it from there and you should be on the right track
How are we supposeed to know the differential of Cotx ?
y = 1/tanx

can I use the quotient rule to find ... =cosec^2 x?
13. I think it's edexcel ?
14. (Original post by Maturity)
How are we supposeed to know the differential of Cotx ?
y = 1/tanx

can I use the quotient rule to find ... =cosec^2 x?
If you're talking about edexcel you'll not be expected to learn it. It will be given to you in the formula booklet.
15. I think its edexcel too, all given in the formula booklet except sinx and cosx
16. (Original post by FZka)
If you're talking about edexcel you'll not be expected to learn it. It will be given to you in the formula booklet.
AQA, no I looked for it and it is not in the given formulae to learn.
But I was wondering how would you actually go about solving it? Anyways.
17. (Original post by Maturity)
How are we supposeed to know the differential of Cotx ?
y = 1/tanx

can I use the quotient rule to find ... =cosec^2 x?
Here you go. Check C3.
Attached Images
18. maths-GCE-formula-book.pdf (548.1 KB, 41 views)
19. (Original post by Maturity)
AQA, no I looked for it and it is not in the given formulae to learn.
But I was wondering how would you actually go about solving it? Anyways.

ohh just saw the message. after i uploaded the formula book.
20. f(x) = cot(x) + 3

First find f'(x)

f'(x) = (d/dx) (cox)(sinx)^-1
f'(x) = -sin(x)(sin(x))^-1 - (sinx)^-2(cosx)^2
f'(x) = -1 - cot^2(x)

then f(x) + f'(x) = 0 is the same as...

cot^2(x) - cot(x) - 2 = 0

then you just have a quadratic.

(cotx - 2)(cotx + 1) = 0

cot(x) = 2 or cot(x) - 1

x = arctan(0.5) or x = arctan(-1)

then you can find all solutions in the given range.

Hope this helps.
21. (Original post by FZka)
ohh just saw the message. after i uploaded the formula book.
Thank you anyway .

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