- #1

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## Homework Statement

I have the attached problem ,and Show my answer and solution,

My answer is 3.14*(15 -8ln4)

While book answer 3.14(15+8ln)

Can any one point to the mistake I made???

- Thread starter keewansadeq
- Start date

- #1

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I have the attached problem ,and Show my answer and solution,

My answer is 3.14*(15 -8ln4)

While book answer 3.14(15+8ln)

Can any one point to the mistake I made???

- #2

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Your calculation looks correct to me. I don't see any obvious errors.

- #3

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The one thing I noticed is the answer from the book would be right if you take the negative away from the 1.

I'm guessing the book has a typo.

- #4

Mark44

Mentor

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If this is actually the book's answer, it makes no sense without something following ln.While book answer 3.14(15+8ln)

- #5

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Absolutely right, but draw the function, you will see that volume is almost as his answer(using geometry not integration),and that why I am confused.

- #6

Ray Vickson

Science Advisor

Homework Helper

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If you think that the book's answer ##\pi (15+8 \ln(4))## is close to the correct answer you are very much mistaken. The correct answer is yours, except that the approximation ##\pi = 3.14## is way too crude---just leave it as symbolic ##\pi## until it comes time to evaluate the final answer numerically. Look at the following:

Absolutely right, but draw the function, you will see that volume is almost as his answer(using geometry not integration),and that why I am confused.

[tex]\begin{array}{rccc}

\pi (15 - 8 \ln\,4 )& \doteq & 12.28251236 & \doteq & 12.3 \\

\pi (15 + 8 \ln\,4 ) & \doteq & 81.96526726 & \doteq & 82.0 \\

\pi 15 & \doteq & 47.12388981 & \doteq & 47.1

\end{array}

[/tex]

- #7

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You are right his answer is not logical,If you think that the book's answer ##\pi (15+8 \ln(4))## is close to the correct answer you are very much mistaken. The correct answer is yours, except that the approximation ##\pi = 3.14## is way too crude---just leave it as symbolic ##\pi## until it comes time to evaluate the final answer numerically. Look at the following:

[tex]\begin{array}{rccc}

\pi (15 - 8 \ln\,4 )& \doteq & 12.28251236 & \doteq & 12.3 \\

\pi (15 + 8 \ln\,4 ) & \doteq & 81.96526726 & \doteq & 82.0 \\

\pi 15 & \doteq & 47.12388981 & \doteq & 47.1

\end{array}

[/tex]

Thanks

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