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Indefinite integral problem

by gyza502
Tags: e^x, help a noob, indefinite integral
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gyza502
#1
Nov10-12, 02:45 AM
P: 4
1. The problem statement, all variables and given/known data
F(x)= (3^x)(e^x)dx



2. Relevant equations
F(u)=U^n=(U^(n+1))/n+1


3. The attempt at a solution
I said it equaled:
((3^(x+1))/(x+1))(e^x)
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vela
#2
Nov10-12, 03:00 AM
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Your post doesn't make sense. Why don't you write things out using normal notation so we don't have to guess as to what you mean?
HallsofIvy
#3
Nov10-12, 07:38 AM
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You've made a number of very basic errors. First, and this may be what Vela was complaining about, it doesn't make sense to have a function equal to an integrand- what you meant to say, instead of [itex]F(x)= 3^xe^xdx[/itex] was that the integrand was [itex]3^x e^x dx[/itex] or, equivalently that you were trying to find [itex]F(x)= \int 3^xe^x dx[/itex].

You make the same kind of error when you write "F(u)=U^n=(U^(n+1))/n+1". [itex]U^{n+1}/(n+1)[/itex] is the integral of [itex]U^n[/itex], they are not equal. (Oh, and two minor things- "u" and "U" are not interchangeable and what you wrote, U^(n+1)/n+ 1 is equal to (U^(n+1)/n)+ 1, not U^(n+1)/(n+1).)

Most importantly, that "power rule" does not apply here. It applies to the variable to a constant power and what you have here is a constant to a variable power. And, of course, you cannot simply multiply by [itex]e^x[/itex] as if it were a constant.

Instead, use the fact that [itex]3^x= e^{ln 3^x}= e^{xln(3)}[/itex] and write the integral, [itex]\int 3^xe^x dx[/itex], as [itex]\int e^{x ln(3)}e^x dx= \int e^{x ln(3)+ x}dx= \int e^{x(ln(3)+ 1)}dx[/itex].

Now, do you know how to integrate [itex]\int e^{ax}dx[/itex]?

gyza502
#4
Nov12-12, 05:51 AM
P: 4
Indefinite integral problem

no i do not. can you tell me please?
HallsofIvy
#5
Nov12-12, 06:06 AM
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This isn't for a class? Do you know the derivative of eax?
gyza502
#6
Nov12-12, 10:18 PM
P: 4
yes it is for a class. But, it is a practice problem. We haven't seen similar problems to this, so i am quite lost at the moment. I know the derivative of e^x is e^x.
SammyS
#7
Nov12-12, 11:50 PM
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Quote Quote by gyza502 View Post
yes it is for a class. But, it is a practice problem. We haven't seen similar problems to this, so i am quite lost at the moment. I know the derivative of e^x is e^x.
Do you know the chain rule ?

If so, use it ti find the derivative of eax, a being a constant.


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