- #1
ross1219
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Homework Statement
Find the area of the surface generated by revolving the curve
Homework Equations
x = 5(cos3t), y = 5(sin3t), 0 ≤ t ≤ [itex]\pi[/itex], about the y axis.
The Attempt at a Solution
x' = -15(cos2t)(sin t)
y' = 15(sin2t)(cos t)
(I think this forms the top part of an astroid)
[x'(t)]2 = 225(cos4t)(sin2t)
[y'(t)]2 = 225(sin4t)(cos2t)
S = 2[itex]\pi[/itex][itex]\int[/itex](from 0 to [itex]\pi[/itex]/2) 5(cos3t) sqrt[225(cos4t)(sin2t) + 225(sin4t)(cos2t)] dt
S = 2[itex]\pi[/itex][itex]\int[/itex](from 0 to [itex]\pi[/itex]/2) 5(cos3t) sqrt[225(cos2t)(sin2t)*(cos2t + sin2t)] dt
S = 2[itex]\pi[/itex][itex]\int[/itex](from 0 to [itex]\pi[/itex]/2) 5(cos3t) sqrt[225(cos2t)(sin2t)(1)] dt
S = 2[itex]\pi[/itex][itex]\int[/itex](from 0 to [itex]\pi[/itex]/2) 5(cos3t)*15(cos t)(sin t) dt
S = 150[itex]\pi[/itex][itex]\int[/itex](from 0 to [itex]\pi[/itex]/2) (cos4t)(sin t) dt
S = -150[itex]\pi[/itex][itex]\int[/itex](from 0 to [itex]\pi[/itex]/2) (cos4t)(-sin t) dt
S = -150[itex]\pi[/itex] [(cos5t) / 5] from 0 to [itex]\pi[/itex]/2
S = -150[itex]\pi[/itex] (0 - 1/5)
S = 30[itex]\pi[/itex] units2
Please excuse my formatting. My basic question is, was I correct to take the integral only from 0 to [itex]\pi[/itex]/2, even though the interval given for the plane curve is from 0 to [itex]\pi[/itex]?
My secondary question is, do you see anything else I messed up?
Thank you much.
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