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# C3 tricky question

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Why bother with a post grad course - waste of time? 17-10-2016

1. Hi, Just need some help

Part C of the question indicates f(t)=ke^t, and that means 3-2e^-x = ke^t. Could someone explain why?
2. It doesn't? It doesn't say they are equal to each other anywhere as far as I can tell

Or are you looking at the mark scheme?
3. (Original post by Fanatical Geek)
It doesn't? It doesn't say they are equal to each other anywhere as far as I can tell

Or are you looking at the mark scheme?
Had no clue what to do with the question so I looked it up

Mark scheme:
4. (Original post by suibster)

Hi, Just need some help

Part C of the question indicates f(t)=ke^t, and that means 3-2e^-x = ke^t. Could someone explain why?
In this situation x=t so f(x) = 3-2e^-x can be written as f(t) = 3-2e^-t

As such you can write f(t) = ke^t as 3-2e^-t = ke^t

I think...
5. (Original post by Narxs)
In this situation x=t so f(x) = 3-2e^-x can be written as f(t) = 3-2e^-t

As such you can write f(t) = ke^t as 3-2e^-t = ke^t

I think...
Thanks.. The answer makes sense but I don't understand why you would make x=t at the first place.
6. (Original post by suibster)
Thanks.. The answer makes sense but I don't understand why you would make x=t at the first place.
The function f is constant for this question as it has not been redefined, (i.e. saying f: y -> y^3 +3) the only thing changing is the variable you sub into it.
7. (Original post by Narxs)
The function f is constant for this question as it has not been redefined, (i.e. saying f: y -> y^3 +3) the only thing changing is the variable you sub into it.
I see, so you're saying that you've just decided to make f(x) = f(t) and just sub it in for no reason?
8. if it says f(t) that means you change the x to t, same as if it were to say f(2) you would sub 2 into the x.

After that you just times all by e^t (since you can just write the 2e^-t as 2/e^t) to make it into a positive which then gives you a quadratic equation.

from there its just straight forward
9. (Original post by suibster)

Hi, Just need some help

Part C of the question indicates f(t)=ke^t, and that means 3-2e^-x = ke^t. Could someone explain why?
What paper is this?
What paper is this?
JAN 2014 IAL C3 edexcel
11. thank you!
12. Multiply each term by e^t which will then give you a quadratic.
Because it only has 1 real solution set the discriminant (b^2 - 4ac) equal to 0 and find k. Hope this helps! (:
13. (Original post by suibster)

Hi, Just need some help

Part C of the question indicates f(t)=ke^t, and that means 3-2e^-x = ke^t. Could someone explain why?
what paper is this from? Just curious.
14. (Original post by suibster)
JAN 2014 IAL C3 edexcel
(Original post by XxKingSniprxX)
what paper is this from? Just curious.
...
15. (Original post by blahblah21)
Multiply each term by e^t which will then give you a quadratic.
Because it only has 1 real solution set the discriminant (b^2 - 4ac) equal to 0 and find k. Hope this helps! (:

thank you! My question is how do you know that you'd make f(t) = f(x) and not just to work out the question f(t)= ke^t on its own?
16. (Original post by suibster)
thank you! My question is how do you know that you'd make f(t) = f(x) and not just to work out the question f(t)= ke^t on its own?
Oh, it's like saying f(3) you would sub in 3 for x. Same principle applies sub in t for x. Hope this kinda helps (:

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