Error in Fitzpatrick and worked solutions? (1 Viewer)

rawrence · Mar 23, 2011

So I was doing a volumes question and I didn't get the right answer (from the textbook) so I checked this guy's full worked solutions and he got the same answer:

What I don't understand is the line from 2pi [5x³/3 - x^4/4 etc.....]

and subbing in 4 gives it 4³/3...

is this a subbing in error or have I don't something stupid?

EDIT: Oh. it's actually written by fitzpatrick, that would explain the same answers

EDIT: can anyone confirm my answer 121pi/6? the question is asking area underneath y=(x-2)² and y=x rotated around y axis using cylindrical shells

Drongoski · Mar 24, 2011

I just saw your post @ 12 midnight,

Have worked out - I got 45 pi/2 like Fitzpatrick. Doesn't mean I'm right but certainly means we both got the same answer.

HyperComplexxx · Mar 24, 2011

$\delta V = \pi [(x+\delta x)^2 - x^2](x-(x-2)^2)$

$= \pi (2x\delta x)(5x - x^2 - 4)$

$V = 2\pi \int_{1}^{4}(5x^2 -x^3 - 4x ) \delta x$

$= 2\pi [\frac{5x^3}{3} - \frac{x^4}{4} - 2x^2 ]_{1}^{4}$

$=2\pi [(\frac{\mathbf{5}(4)^3}{3}-\frac{4^4}{4}-2(4)^2)-(\frac{5(1)^3}{3}-\frac{(1)^4}{4}-2(1)^4)]$

$= \frac{45 \pi }{2} units^3$

rawrence · Mar 24, 2011

Oh, I don't know what I did wrong, got the answer now by redoing

Error in Fitzpatrick and worked solutions? (1 Viewer)

rawrence

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Drongoski

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HyperComplexxx

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rawrence

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