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# integration watch

1. Hi guys ,
I'm trying to integrate -4e^-t , isn't this in the form of y=e^f(x) hence dy/dx=f'(x)e^f(x) ? the answer comes out as +4e^-t but somehow i just can't get to that any help would be much appreciated
2. (Original post by Alen.m)
Hi guys ,
I'm trying to integrate -4e^-t , isn't this in the form of y=e^f(x) hence dy/dx=f'(x)e^f(x) ? the answer comes out as +4e^-t but somehow i just can't get to that any help would be much appreciated
What answer did you get? And could you post your working/thoughts to explain how you got it?
3. (Original post by notnek)
What answer did you get? And could you post your working/thoughts to explain how you got it?
first i diffrentiated -t which gave me -t^2/2 and then put in the form of dy/dx=f'(x)e^f(x) which again gave me -4(-t^2/2 e^-t) and this is just far away from what the text book said
4. (Original post by Alen.m)
first i diffrentiated -t which gave me -t^2/2 and then put in the form of dy/dx=f'(x)e^f(x) which again gave me -4(-t^2/2 e^-t) and this is just far away from what the text book said
Do you know that ? If you differentiate the result you obtained, it does not give your initial function.

Recall that .
5. (Original post by Alen.m)
first i diffrentiated -t which gave me -t^2/2 and then put in the form of dy/dx=f'(x)e^f(x) which again gave me -4(-t^2/2 e^-t) and this is just far away from what the text book said
You have integrated when you should have differentiated

6. (Original post by Alen.m)
first i diffrentiated -t which gave me -t^2/2 and then put in the form of dy/dx=f'(x)e^f(x) which again gave me -4(-t^2/2 e^-t) and this is just far away from what the text book said
Differentiating -t with respect to t will give you -1.
7. (Original post by aymanzayedmannan)
Do you know that ? If you differentiate the result you obtained, it does not give your initial function.

Recall that .
To be honest the text bok that im revising has got nothing about the formula you nust mentioned and that was why i kept trying wrong ways i guess but yours absoultely makes sense thanks
8. (Original post by Alen.m)
To be honest the text bok that im revising has got nothing about the formula you nust mentioned and that was why i kept trying wrong ways i guess but yours absoultely makes sense thanks
This formula dy/dx=f'(x)e^f(x) does work.

You just made the mistake of integrating -t when you should have differentiated it to give f'(x) = -1.
9. (Original post by notnek)
This formula dy/dx=f'(x)e^f(x) does work.

You just made the mistake of integrating -t when you should have differentiated it to give f'(x) = -1.
Well actually the question is on M2 where you have to integrate certain expression to get to another expeesion for displacement, acceleration and velocity . Im half sure that i had to integrate certain expression which had -4e^-t in it as well to get to another expression. I'll send you the full question once i got home

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