Ok so when you integrate (e^(-x^4))(-4x^3) you get e^(-x^4) due to the u substitution of du = -4x^3 dx....but after you sub in you don't integrate the e^(-x^4)..why is that? I found a problem (e^-x)/(1-e^-x) where after you sub in for du = -e^(-x)dx and get -1/(1-e^-x) you still integrate why why why!
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In the first problem you didn't integrate again because you were done. You had eliminated the integral operation and had an answer.
I suspect, since I can't see the solution you are looking at, that in the second problem you weren't done after the first substitution. The integral was not yet eliminated. So, you had to continue.
See these references and choose show steps. Compare the two solutions and see if they match what you were seeing.
http://www.wolframalpha.com/input/?i=int…
http://www.wolframalpha.com/input/?i=int…
In the first problem you didn't integrate again because you were done. You had eliminated the integral operation and had an answer.
I suspect, since I can't see the solution you are looking at, that in the second problem you weren't done after the first substitution. The integral was not yet eliminated. So, you had to continue.
See these references and choose show steps. Compare the two solutions and see if they match what you were seeing.
http://www.wolframalpha.com/input/?i=int…
http://www.wolframalpha.com/input/?i=int…
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Why wouldn't you integrate? Are you confused on how to do u-substitutions?
-1/(1 - e^(-x)) = -1/u du is very easy to integrate as -ln(u) = -ln(1 - e^(-x))
-1/(1 - e^(-x)) = -1/u du is very easy to integrate as -ln(u) = -ln(1 - e^(-x))
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are you sure you do not integrate ???? I am pretty sure you should.