# volumes of revolutionWatch

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#1
so, for b, I've chosen to revolves the curve around the y axis, with the top curves volume being measures from 12 to 7, and the bottom one 8 to 7. I've subtracted the volume of the bottom curve from the top. however this doesnt give me the right answer.
Last edited by FurtherMaths2020; 5 days ago
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5 days ago
#2
You've rotated around the wrong axis. The question makes it clear that you need to rotate around the x-axis.
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#3
(Original post by dextrous63)
You've rotated around the wrong axis. The question makes it clear that you need to rotate around the x-axis.
ok, I see that but shouldnt it give me the right/same answer?
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5 days ago
#4
(Original post by FurtherMaths2020)
ok, I see that but shouldnt it give me the right/same answer?
No. It will create an entirely different shape (ie a ring, and not some kind of weird eggshell thingy!!)
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#5
(Original post by dextrous63)
No. It will create an entirely different shape (ie a ring, and not some kind of weird eggshell thingy!!)
touche
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5 days ago
#6
(Original post by FurtherMaths2020)
ok, I see that but shouldnt it give me the right/same answer?
Definetly not.

think about the volume your creating as you pull the 2D cross section around the axis.
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5 days ago
#7
You might find it useful to develop your Geogebra/Autograph skills to draw 3D shapes. Good fun (in a strange sort of way). Also highly useful for drawing parametric curves, amongst other things.
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#8
(Original post by dextrous63)
You might find it useful to develop your Geogebra/Autograph skills to draw 3D shapes. Good fun (in a strange sort of way). Also highly useful for drawing parametric curves, amongst other things.
I totally see it now - I hadn't had my morning coffee and was feeling a little sluggish.
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