C3 tricky question

suibster

Hi, Just need some help :smile:

Part C of the question indicates f(t)=ke^t, and that means 3-2e^-x = ke^t. Could someone explain why?

Reply 1

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?

Reply 2

suibster

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: Screen Shot 2016-06-19 at 21.29.08.png

Reply 3

Narxs

Original post by suibster

Hi, Just need some help :smile:

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...

Reply 4

suibster

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.

Reply 5

Narxs

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.

Reply 6

suibster

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?

Reply 7

PhoenixBlaze123

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

Reply 8

BusinessLover18

Original post by suibster

Hi, Just need some help :smile:

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?

Reply 9

suibster

Original post by BusinessLover18

What paper is this?

JAN 2014 IAL C3 edexcel

thank you!

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! (:

Reply 12

XxKingSniprxX

Original post by suibster

Hi, Just need some help :smile:

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. :tongue:

Reply 13

Zacken

Original post by suibster

JAN 2014 IAL C3 edexcel

Original post by XxKingSniprxX

what paper is this from? Just curious. :tongue:

...

Reply 14

suibster

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! (: