Hello, can anyone tell me how I can show that this

1. Nov 2, 2011

neom

Hello, can anyone tell me how I can show that this diverges

$$\int_{-1}^0\frac{e^x}{x+1}dx$$

I'm out of ideas

2. Nov 2, 2011

lavinia

Re: Divergence

When in doubt try integration by parts

3. Nov 2, 2011

neom

Re: Divergence

Yes, of course. Thanks a lot!

4. Nov 2, 2011

neom

Re: Divergence

No, it's not working, I really can't evaluate this integral. Is there another way to see that it diverges?

5. Nov 2, 2011

homeomorphic

Re: Divergence

As a warm-up, why does the integral of 1/x from 0 to 1 diverge?

It's not too different from that. Maybe use the comparison test, since you can bound e^x from below by some number greater than zero.

6. Nov 2, 2011

neom

Re: Divergence

Because $$\lim_{x\to0}\frac1{x}$$ diverges? I'm not sure. What's the comparison test?

7. Nov 2, 2011

disregardthat

Re: Divergence

Find a lower bound for the integrand on [-1,0], that is, a function that is less than your integrand on [1,0]. (technically the absolute value of the integrand, but in this case it is positive)

Hint: what is the minimal value of e^x on [-1,0] ?

Then you can use the comparison test to see whether the integral of the lower bound diverges.

8. Nov 2, 2011

lavinia

Re: Divergence

Integrating by parts gives

$$\frac{e^x}{x+1}|^{0}_{-1}+ \int_{-1}^0\frac{e^x}{(x+1)^2}dx$$

The first term diverges. If the second term is finite then the whole thing diverges. What if the second term diverges?

Last edited: Nov 2, 2011
9. Nov 6, 2011

Woopy

Re: Divergence

If one part diverges, then the whole thing diverges

10. Nov 6, 2011

lurflurf

Re: Divergence

^No a sum of two divergent terms can converge such as (1/x-1/x) which converges even though each part diverges. For this problem show that
$$\int_{-1}^0\frac{e^x+a}{x+1}dx$$
can converge only when a =-e-1
and in particular not when a=0

11. Nov 7, 2011

dimension10

Re: Divergence

Because ln(0) is slightly undefined?

12. Nov 11, 2011

vikiabst

Re: Divergence

integral -1 to 0 (e^x)/1+x>e^-2[integral -1 to 0 1/(1+x)]

The second term diverges so first term diverges.

13. Nov 11, 2011

lavinia

Re: Divergence

The argument i was making was not that. The first term, is easily seen to diverge since the numerator is finite and the denominator blows up at -1. It diverges to +infinity. If the second term is finite then the sum must diverge. But since the second term is positive, if it diverges, then it also diverges to plus infinity so the sum of the two must diverge to plus infinity.