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# Edexcel C3 (20/06) Revision Thread watch

1. when you differentiate 1/lna why does it stay as 1/lna?
2. Also remember:
(a/b)/c = a/(bc)
a/(b/c) = ac/b

If you have a fraction at the bottom, you flip it (primary school style), and multiply by the top. If you get confused, try it out with real numbers...
3. (Original post by mockel)
Also remember:
(a/b)/c = a/(bc)
a/(b/c) = ac/b

If you have a fraction at the bottom, you flip it (primary school style), and multiply by the top. If you get confused, try it out with real numbers...
safe Mockel! now i wont get so confused.
4. (Original post by Gaz031)
y=(lna)^-1
y^-1=(lna)
(-y^-2)(dy/dx)=1/a
(dy/dx)=(-1/a)(y^2)
(dy/dx)=(-1/a)(lna)^-2
(dy/dx)=-1/[a(lna)^2], which isn't the same as 1/lna.
Hmm in the edexcel C3 paper A5 it says it stays the same.

Ok let me say the whole question y = [1/lna] * [lnx]

When u differentiate it, the answer says it is 1/xlna, and it assumes that when you differetiate 1/lna, it stays the same.
5. (Original post by sb1986)
Hmm in the edexcel C3 paper A5 it says it stays the same.

Ok let me say the whole question y = [1/lna] * [lnx]

When u differentiate it, the answer says it is 1/xlna, and it assumes that when you differetiate 1/lna, it stays the same.
Actually in my previous post i meant to type x instead of a. I didn't realise you meant a constant rather than a variable, sorry.
Differentiating 1/lna gives 0, as 1/lna is constant.
6. 1/lna is just a constant.
Differentiating (1/lna)(lnx), is just (1/lna)(1/x) = (1/xlna)

It's like if you had to do d/dx[3x]. The answer would be 3, since the 3 is similarly, just a constant.
7. yeh, i keep doing these silly mistakes, lol. Thanks
8. Can someone explain the method of finding the range, i know that for a quadratic you can just complete the square, but what about other functions? I know it involves differentiating.
9. Thanks Featherflare, Seth & nTrik. Seems like I forgot to multiply 0.8x by -1.
10. Has anyone managed sin 3x = cos x yet...there was an error in my previous working but the answers I got satisfied the initial equation which is quite a coincidence...
11. (Original post by sb1986)
Can someone explain the method of finding the range, i know that for a quadratic you can just complete the square, but what about other functions? I know it involves differentiating.
Anyone?
12. Nope I think sin3x = cos x is not on the syllabus is it?
13. (Original post by sb1986)
Anyone?
Depends on the example - usually drawing a graph is the best first move

Do you have any specific examples that are troubling you?
14. (Original post by sb1986)
Anyone?
Just look at the domain and look at the function, that's how I do it. punch numbers into the calculator to figure out how the curve works and you can generally get it from that.
15. (Original post by sb1986)
Can someone explain the method of finding the range, i know that for a quadratic you can just complete the square, but what about other functions? I know it involves differentiating.
For most of them, I draw a graph. ;o
16. (Original post by mik1w)
Nope I think sin3x = cos x is not on the syllabus is it?
I'm not sure..someone earlier on in the post said it was on a paper..but I can't seem to solve it properly anyway
17. More like sin3x ≠ cosx am I right?
18. (Original post by sb1986)
Can someone explain the method of finding the range, i know that for a quadratic you can just complete the square, but what about other functions? I know it involves differentiating.
Try drawing a graph as its usually the easiest way to figure it out. The range is the values f(x) or y on the graph can take.
Alternatively, you can sometimes just figure it out by looking at the domain (this is the values which x can take in the function) and punching numbers into the function.
If the function is exponential, (eg f(x) = e^a +2 ), then as e^a must be above zero, the range must be f(x) > 0 + 2, f(x) >2

Hope that helped
19. (Original post by ntrik)
More like sin3x ≠ cosx am I right?
Yea iv done the unthinkable.. given up. I dont think it can be done with c1- c4 knowledge which is all i have
20. (Original post by Featherflare)
Try drawing a graph as its usually the easiest way to figure it out. The range is the values f(x) or y on the graph can take.
Alternatively, you can sometimes just figure it out by looking at the domain (this is the values which x can take in the function) and punching numbers into the function.
If the function is exponential, (eg f(x) = e^a +2 ), then as e^a must be above zero, the range must be f(x) > 0 + 2, f(x) >2

Hope that helped
Thanks.

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