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### Section 7.9 : Comparison Test for Improper Integrals

3. Use the Comparison Test to determine if the following integral converges or diverges.

$\int_{4}^{\infty }{{\frac{{{{\bf{e}}^{ - y}}}}{y}\,dy}}$

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Hint : Start off with a guess. Do you think this will converge or diverge?
Start Solution

The first thing that we really need to do here is to take a guess on whether we think the integral converges or diverges.

We need to be a little careful with the guess for this problem. We might be tempted to use the fact from the previous section to guess diverge since the exponent on the $$y$$ in the denominator is $$p = 1 \le 1$$ .

That would be incorrect however. Recall that the fact requires a constant in the numerator and we clearly do not have that in this case. In fact what we have in the numerator is $${{\bf{e}}^{ - y}}$$ and this goes to zero very fast as $$y \to \infty$$ and so there is a pretty good chance that this integral will in fact converge.

Be careful from this point on! One of the biggest mistakes that many students make at this point is to say that because we’ve guessed the integral converges we now know that it converges and that’s all that we need to do and they move on to the next problem.

Another mistake that students often make here is to say that because we’ve guessed that the integral converges they make sure that the remainder of the work in the problem supports that guess even if the work they do isn’t correct.

All we’ve done is make a guess. Now we need to prove that our guess was the correct one. This may seem like a silly thing to go on about, but keep in mind that at this level the problems you are working with tend to be pretty simple (even if they don’t always seem like it). This means that it will often (or at least often once you get comfortable with these kinds of problems) be pretty clear that the integral converges or diverges.

When these kinds of problems arise in other sections/applications it may not always be so clear if our guess is correct or not and it can take some real work to prove the guess. So, we need to be in the habit of actually doing the work to prove the guess so we are capable of doing it when it is required.

The hard part with these problems is often not making the guess but instead proving the guess! So let’s continue on with the problem.

Hint : Now that we’ve guessed the integral converges do we want a larger or smaller function that we know converges?
Show Step 2

Recall that we used an area analogy in the notes of this section to help us determine if we want a larger or smaller function for the comparison test.

We want to prove that the integral converges so if we find a larger function that we know converges the area analogy tells us that there would be a finite (i.e. not infinite) amount of area under the larger function.

Our function, which would be smaller, would then also have a finite amount of area under it. There is no way we can have an infinite amount of area inside of a finite amount of area!

Note that the opposite situation does us no good. If we find a smaller function that we know converges (and hence will have a finite amount of area under it) our function (which is now larger) can have either a larger finite amount of area or an infinite area under it.

In other words, if we find a smaller function that we know converges this will tell us nothing about our function. However, if we find a larger function that we know converges this will force our function to also converge.

Therefore we need to find a larger function that we know converges.

Show Step 3

Okay, now that we know we need to find a larger function that we know converges.

So, let’s start with the function from the integral. It is a fraction and we know that we can make a fraction larger by making the denominator smaller. From the limits on the integral we can see that,

$y > 4$

Therefore, we have,

$\frac{{{{\bf{e}}^{ - y}}}}{y} < \frac{{{{\bf{e}}^{ - y}}}}{4}$

since we replaced the denominator with something that we know is smaller.

Show Step 4

Finally, we will need to prove that,

$\int_{4}^{\infty }{{{{\bf{e}}^{ - y}}\,dy}}$

converges. However, after the previous section that shouldn’t be too difficult. Here is that work.

$\int_{4}^{\infty }{{\frac{1}{4}{{\bf{e}}^{ - y}}\,dy}} = \mathop {\lim }\limits_{t \to \infty } \int_{4}^{t}{{\frac{1}{4}{{\bf{e}}^{ - y}}\,dy}} = \mathop {\lim }\limits_{t \to \infty } \left. {\left( { - \frac{1}{4}{{\bf{e}}^{ - y}}} \right)} \right|_4^t = \mathop {\lim }\limits_{t \to \infty } \left( { - \frac{1}{4}{{\bf{e}}^{ - t}} + \frac{1}{4}{{\bf{e}}^{ - 4}}} \right) = \frac{1}{4}{{\bf{e}}^{ - 4}}$

The limit existed and was finite and so we know that,

$\int_{4}^{\infty }{{\frac{1}{4}{{\bf{e}}^{ - y}}\,dy}}$

converges.

Therefore, because the function in this integral is larger than the function in the original integral the Comparison Test tells us that,

$\int_{4}^{\infty }{{\frac{{{{\bf{e}}^{ - y}}}}{y}\,dy}}$

must also converge.