# Review Problems for Test 2

Math 211

10-12-2017

These problems are provided to help you study. The presence of a problem on this sheet does not imply that a similar problem will appear on the test. And the absence of a problem from this sheet does not imply that the test will not have a similar problem.

1. Find the area of the region bounded by the graphs of and .

2. Find the area of the region between and from to .

3. Find the area of the region bounded by and , between the first two intersections of the curves for which .

4. The region bounded by and the x-axis is revolved about the x-axis. Find the volume of the solid that is generated.

5. Consider the region in the x-y plane bounded by , the line , and the line . Find the volume generated by revolving the region:

6. The base of a solid is the region in the x-y plane bounded by the curves and . The cross-sections of the solid perpendicular to the x-y plane and the x-axis are isosceles right triangles with one leg in the x-y plane. Find the volume of the solid.

7. The base of a solid is the region in the x-y-plane bounded above by the line and below by the parabola . The cross-sections in planes perpendicular to the y-axis are squares having one edge in the x-y-plane. Find the volume of the solid.

8. The region which lies above the x-axis and below the graph of , , is revolved about the x-axis. Find the volume of the solid which is generated.

Hint:

9. A force of 8 pounds is required to extend a spring 2 feet beyond its unstretched length.

(a) Find the spring constant k.

(b) Find the work done in stretching the spring from 2 feet beyond its unstretched length to 3 feet beyond its unstretched length.

10. The base of a rectangular tank is 2 feet long and 3 feet wide; the tank is 6 feet high. Find the work done in pumping all the water out of the top of the tank.

11. Write a formula for the n-th term of the sequence, assuming that the terms continue in the "obvious" way.

(a) .

(b) .

12. A sequence is defined recursively by

Write down the first 5 terms of the sequence.

13. Determine whether the sequence for eventually increases, decreases, or neither increases nor decreases.

14. Determine whether the sequence for eventually increases, decreases, or neither increases nor decreases.

15. Is the following sequence bounded? Why or why not?

16. Determine whether the sequence converges or diverges; if it converges, find the limit.

(a) .

(b) .

(c) .

(d) .

(e) .

(f) .

17. A sequence is defined recursively by

Find .

18. If the series converges, find the exact value of its sum; if it diverges, explain why.

(a) .

(b) .

(c)

(d) .

(e) .

(f) .

19. (a) Find the partial fractions decomposition of .

(b) Use (a) to find the sum of the series

20. Find series and such that both series diverge, and:

(a) diverges.

(b) converges.

21. Calvin Butterball notes that , and concludes that the series converges by the Zero Limit Test. What's wrong with his reasoning?

22. If the series , converges, does the series converge?

23. Does the series converge?

24. Determine whether the series converges or diverges: .

25. Determine whether the series converges or diverges: .

26. Determine whether the series converges or diverges: .

27. Determine whether the series converges or diverges: .

# Solutions to the Review Problems for Test 2

1. Find the area of the region bounded by the graphs of and .

The curves intersect at and at :

is the top curve and is the bottom curve. Hence, the area is

2. Find the area of the region between and from to .

The curves intersect at and :

Since the curves cross between 0 and 5, I will need two integrals. On the left-hand piece, the top curve is and the bottom curve is . On the right-hand piece, the top curve is and the bottom curve is . The area is

3. Find the area of the region bounded by and , between the first two intersections of the curves for which .

Solve the curve equations simultaneously:

Break the region up into horizontal rectangles. The length of a typical rectangle is . The area is

4. The region bounded by and the x-axis is revolved about the x-axis. Find the volume of the solid that is generated.

The region extends from to . I'll use circular slices. The radius of a typical slice is . The area of a typical slice is

The volume generated is

5. Consider the region in the x-y plane bounded by , the line , and the line . Find the volume generated by revolving the region:

(a)

Since the solid has no "holes" or "gaps" in its interior, I can use circular slices. The radius of a slice is , so the volume is

(b)

I'll use cylindrical shells. The height is , and the radius is . The volume is

Here's the work for part of the integral:

(c)

I'll use cylindrical shells. Since gives , the height is , and the radius is . The vertical limits on the region are and . The volume is

Here is how I did two of the pieces of the integral:

6. The base of a solid is the region in the x-y plane bounded by the curves and . The cross-sections of the solid perpendicular to the x-y plane and the x-axis are isosceles right triangles with one leg in the x-y plane. Find the volume of the solid.

The first picture shows the base of the solid. The second picture shows three typical triangular slices standing on the base.

gives , so and or . Therefore, the base of the solid extends from to .

The leg of a triangular slice has length . Hence, the area of a triangular slice is . The volume is

7. The base of a solid is the region in the x-y-plane bounded above by the line and below by the parabola . The cross-sections in planes perpendicular to the y-axis are squares having one edge in the x-y-plane. Find the volume of the solid.

The first picture shows the base of the solid. The second picture shows three typical square slices standing on the base.

The thickness of a typical slice is in the y-direction, so I'll use y as my variable. Solving for x gives .

The side of a square slice extends from to , so its length is . The area of a typical square slice is . Hence, the volume is

8. The region which lies above the x-axis and below the graph of , , is revolved about the x-axis. Find the volume of the solid which is generated.

Chop the solid up into circular slices perpendicular to the x-axis. The thickness of a typical slice is . The radius of a slice is . The volume is

Compute the first integral:

(I used the fact that .)

Similarly,

The volume is .

9. A force of 8 pounds is required to extend a spring 2 feet beyond its unstretched length.

(a) Find the spring constant k.

(b) Find the work done in stretching the spring from 2 feet beyond its unstretched length to 3 feet beyond its unstretched length.

(a)

(b) Since , I have . Hence, the work done is

10. The base of a rectangular tank is 2 feet long and 3 feet wide; the tank is 6 feet high. Find the work done in pumping all the water out of the top of the tank.

Divider the water up into rectangular slabs parallel to the base. Let y denote the height of a slab above the base.

The volume of a typical slab is , so the weight is . (The density of water is 62.4 pounds per cubic foot.)

A slab at height y must be lifted a distance of to get to the top of the tank. Therefore, the work done in lifting the slab is . The total work is

11. Write a formula for the n-th term of the sequence, assuming that the terms continue in the "obvious" way.

(a) .

(b) .

(a)

(b)

12. A sequence is defined recursively by

Write down the first 5 terms of the sequence.

13. Determine whether the sequence for eventually increases, decreases, or neither increases nor decreases.

Let . Then

Hence, the sequence increases.

14. Determine whether the sequence for eventually increases, decreases, or neither increases nor decreases.

The terms are

In fact, . Hence, the sequence neither increases nor decreases.

15. Is the following sequence bounded? Why or why not?

The even-numbered terms have the form n for , and . Hence, the sequence is not bounded.

16. Determine whether the sequence converges or diverges; if it converges, find the limit.

(a)

(b)

(c)

(d)

(e) .

(f) .

(a) Since is a geometric sequence with ratio ,

(b) Divide the top and bottom by (since is the biggest exponential in the fraction):

I computed the limit using the fact that the following are geometric sequences:

Their ratios are all less than 1, so they go to 0 as .

(c)

I did this by considering the highest powers on the top and bottom; they're both , so I just looked at their coefficients. You could also do this by using L'H\^opital's rule, or by dividing the top and the bottom by .

(d)

(e) Note that is undefined, so I can't take the limit of the terms directly. Instead, I'll use the Squeezing Theorem. I have

Also,

By the Squeezing Theorem, .

(f) Note that

Since , it follows that

17. A sequence is defined recursively by

Find .

Taking the limit on both sides of the recursion equation, I get

I'm allowed to move the limit inside the square root by a standard rule for limits.

Now because both limits represent what the sequence is approaching. So let

Then

Since the sequence consists of positive numbers, it can't have a negative limit. This rules out -3. Therefore,

18. If the series converges, find the exact value of its sum; if it diverges, explain why.

(a) .

(b) .

(c)

(d) .

(e) .

(f) .

(a) The series converges, and

(b) Since the ratio -1.021 is not in the interval , the series diverges. In fact, it diverges by oscillation, as alternate partial sums approach and .

(c) The series converges, and

(d) The series is the sum of two convergent geometric series, so it converges. First,

Next,

Hence,

(e) The series is a convergent geometric series, but is a divergent geometric series, since the ratio is greater than 1. Hence, the given series diverges --- in fact, it diverges to .

(f) Note that

Writing out the first few terms, you can see that the series converges by telescoping:

19. (a) Find the partial fractions decomposition of .

(b) Use (a) to find the sum of the series

(a)

Set : I get , so .

Set : I get , so .

Therefore,

(b)

The second fraction in each pair cancels with the first fraction in the next pair. The only one that isn't cancelled is the very first one: . Therefore,

20. Find series and such that both series diverge, and:

(a) diverges.

(b) converges.

(a) Let and . Then and both diverge, because they're harmonic.

And diverges as well, since it's twice the harmonic series.

(b) Let and . Then diverges because it's the harmonic series, and diverges because it's the negative of the harmonic series.

However, converges, and its sum is 0.

This problem shows that the term-by-term sum of two divergent series can either converge or diverge.

21. Calvin Butterball notes that , and concludes that the series converges by the Zero Limit Test. What's wrong with his reasoning?

The Zero Limit Test says that if the limit of the terms is not 0, then the series diverges. It does not say that if the limit of the terms is 0, then the series converges. (The second statement is called the converse of the first; the converse of a statement is not the same as, or equivalent to, the statement.)

In fact, diverges, because it's a p-series with .

22. If the series , converges, does the series converge?

If the series converges, then the series converges. They only differ in the first 16 terms, and a finite number of terms cannot affect the convergence or divergence of an infinite series.

23. Does the series converge?

The series alternates, but

The causes the terms to oscillate in sign, so

The series diverges by the Zero Limit Test.

24. Determine whether the series converges or diverges: .

The series is the sum of two convergent geometric series; in fact, its sum is

25. Determine whether the series converges or diverges: .

The series on the right are convergent p-series. Hence, the original series converges.

26. Determine whether the series converges or diverges: .

Note that

Hence, the series diverges by the Zero Limit Test.

27. Determine whether the series converges or diverges: .

Since , the series is a convergent p-series.

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