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# Edexcel C3,C4 June 2013 Thread watch

1. (Original post by LegendX)
See attachment
ahhh I was getting stuck at differentiating xlna, but lna is just a constant which is why x -> 1 or rather, x disappears

thanks very much
(is that biology? lol)
2. (Original post by PythianLegume)
Integration finds the total area, and area below the x-axis is considered 'negative area' therefore, if a graph has sections above and below the x-axis, you have to split it into integrals and take the modulus of the negative integral.
Thank you

I see how that would work for something like a sine graph (where it's easier to split the parts of the curve) but what about a graph where limits for both areas (above and below) are the same? How would we split it then?
Yh i get dat bit buh wah if cot3x came up in the exam? would it be 1/3ln[sin(3x)]?
Yeh.
4. (Original post by MedMed12)
its all about the MBE, OBEs at the moment
Surely in a maths exam it's all about the ODE's?

How's everyone feeling for this tomorrow?
5. (Original post by Supes180)
If you swap limits by substitution and you end up with limits that have a different size order, do you still go from high to low/top to bottom? For example turns into Is that right? Or should it be
Keep them the first way.
6. (Original post by Frankster)
y = a^x

dy/dx = a^x lna

y = a^x

ln y = xlna

1/y . dy/dx = ln a

dy/dx = y ln a

(From y = a^x )

Therefore: dy/dx = a^x ln a
can someone explain to me why the x goes in the bolded line?
7. (Original post by Mallika)
Thank you

I see how that would work for something like a sine graph (where it's easier to split the parts of the curve) but what about a graph where limits for both areas (above and below) are the same? How would we split it then?
Well, if the graph is symmetrical above and below the x-axis, you double the answer - all you have found is the area above the x-axis (assuming you didn't get an answer of zero).
8. (Original post by Kreayshawn)
can someone explain to me why the x goes in the bolded line?
It's been differentiated.
9. (Original post by arcturus7)
Surely in a maths exam it's all about the ODE's?

How's everyone feeling for this tomorrow?
haha good one!!
Good I like c4 just so tired
wbu?
10. n an experiment a scientist considered the loss of mass of a collection of picked leaves. The
mass M grams of a single leaf was measured at times t days after the leaf was picked.
The scientist attempted to find a relationship between M and t. In a preliminary model she
assumed that the rate of loss of mass was proportional to the mass M grams of the leaf.
(a) Write down a differential equation for the rate of change of mass of the leaf, using this
model.
(2)
(b) Show, by differentiation, that M = 10(0.98)^t
satisfies this differential equation.

I am really stuck on part b, can anyone help?

I've got this far: dm/dt=-kv
For the diffrentiation part: dM/dt: 10x0.98^(t)xln(0.98)
11. Someone PLEASE post their solution to question 7c from paper E!!! Emailed my teacher and they havent replied -_-
12. (Original post by Tikara)
ahhh I was getting stuck at differentiating xlna, but lna is just a constant which is why x -> 1 or rather, x disappears

thanks very much
(is that biology? lol)
Yeah lol it's biology, I'm running out of paper so using whatever I can find lol haha.
13. Is everyone ready for C4, i can't wait for it to be over :/ It's boring doing the same maths again and again and again and again.....:/
14. An integral a day keeps the doctor away...
15. (Original post by Econ1994)
Someone PLEASE post their solution to question 7c from paper E!!! Emailed my teacher and they havent replied -_-
post the question and ill try and help i dont know what paper E is tho

17. (Original post by PythianLegume)
Well, if the graph is symmetrical above and below the x-axis, you double the answer - all you have found is the area above the x-axis (assuming you didn't get an answer of zero).
Just one more question

What if it wasn't symmetrical? Or would we never such a graph in C4? (There's a question like this in the mixed exercise for integration which is why I'm asking)
Thank you again
18. I haven't done C3 for a year so can anybody think of anybody i should read through thats in c3 that could potentially come up to tomorrow, or something to just remind myself about.
19. (Original post by LegendX)
Look at the attachment.
For the last part, when we got our H value, n u subbed it back in, did u still use 120 as your dv over dt

20. How do you integrate this?

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