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Core 4 Integration (by substitution... Hard) rep+

Hey guys!

I am really stuck on this, so some help would be much appreciated!

By using the substitution t=e^x, show that integral of e^2x/(1+e^x)= integral of t/(1+t).

Thats fine and I have done that, now it says....
Deduce that integral of e^2x/(1+e^x)= e^x-ln(1+e^x)+c

Now I dont know how to remove the denominator when dividing t by (1+t).

If I were to do it normally I WOULD do Tln(1+t)+c
which is e^x.ln (1+e^x)+c

I don't know how they got that answer above of e^x-ln(1+e^x)+c

Help please!!
Reply 1
t/( 1 + t ) is equivalent to ( 1 + t -1 )/( 1 + t ) or

1 - 1/( 1 + t )...
Original post by J DOT A
Hey guys!

I am really stuck on this, so some help would be much appreciated!

By using the substitution t=e^x, show that integral of e^2x/(1+e^x)= integral of t/(1+t).

Thats fine and I have done that, now it says....
Deduce that integral of e^2x/(1+e^x)= e^x-ln(1+e^x)+c

Now I dont know how to remove the denominator when dividing t by (1+t).

If I were to do it normally I WOULD do Tln(1+t)+c
which is e^x.ln (1+e^x)+c

I don't know how they got that answer above of e^x-ln(1+e^x)+c

Help please!!


Reduce the order/degree of the fraction (not sure which term is technically right...anyone?)
Reply 3
Original post by the bear
t/( 1 + t ) is equivalent to ( 1 + t -1 )/( 1 + t ) or

1 - 1/( 1 + t )...



okay so how would I integrate (1 +t -1)/(1+t) ?
This is confusing me:frown:
Reply 4
Original post by J DOT A
okay so how would I integrate (1 +t -1)/(1+t) ?
This is confusing me:frown:


you can integrate 1 and then -1/(1 + t)...

hint: you will need logarithms
Original post by J DOT A
okay so how would I integrate (1 +t -1)/(1+t) ?
This is confusing me:frown:


1+t11+t=111+t\frac{1+t-1}{1+t}=1-\frac{1}{1+t}

Remember that 1xdx=lnx\displaystyle\int\frac{1}{x} dx = ln x
(edited 12 years ago)

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