Homework Help: Did I do this integral right?

1. Feb 15, 2005

UrbanXrisis

$$\int \frac {e^x+4}{e^x}dx =?$$

Here's what I did:
$$\int \frac {e^x+4}{e^x}dx = \int e^{-x}(e^x+4)dx$$
$$\int e^{-x}(e^x+4)dx =\int 1+4e^{-x} = x-\frac {4e^{-x+1}}{x+1}$$

Did I do this correctly? Is there a more simplified answer?

2. Feb 15, 2005

Galileo

Rewriting $\frac{e^x+4}{e^x}=1+4e^{-x}$ was correct.

Check the antiderivative of $e^{-x}$. Your answer is not correct. You can easily check it by differentiating it.

Mind the difference between $x^a$ where the base is the variable and $a^x$ where the base is constant and the exponent is the variable.

3. Feb 15, 2005

UrbanXrisis

since $$\int e^{x} = e^{x}+C$$
then...
$$\int 1+4e^{-x} = x+4e^{-x}+C$$

is that correct?

4. Feb 15, 2005

Jameson

When you differentiate $$x + 4e^{-x} + C$$ you get

$$1 - 4e^{-x}$$ , so the integral is actually

$$\int 1 + 4e^{-x} dx = x - 4e^{-x} + C$$

5. Feb 15, 2005

UrbanXrisis

I dont understand where the negative came from

6. Feb 15, 2005

Jameson

The integral of $$e^x dx = {e^x} + C$$

The integral of $$e^{-x}dx = -e^{-x} + C$$.

Differentiating that answer you find that $$\frac {d}{dx} -e ^{-x} = e^{-x}$$

7. Feb 15, 2005

dextercioby

Think of it as an $e^{u}$ and apply the method of substitution:
$$-x=u$$

Daniel.

P.S.That's how u end up with the minus.

8. Feb 15, 2005

UrbanXrisis

$$\int \frac {e^x}{e^x+4}dx =?$$

Here's what I did:
$$= \int e^{x}(e^x+4)^{-1}dx$$
subsitute:
$$u=e^x+4$$
$$du=e^x dx$$
$$\int u^{-1}du =ln(e^x+4)$$

Did I do this correctly? Is there a more simplified answer?

9. Feb 15, 2005

NateTG

Looks good to me.

10. Feb 15, 2005

dextercioby

Don't forget the constant of integration.

Daniel.