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> Polar Area > Calc 3
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need2passCalcIII
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#1
Jan 31 2016 11:50am
The problem is to find the area of the region enclosed by the loop of the strophoid r=2cos(θ)−sec(θ). I am having trouble figuring out how to get the limits of integration for this one. Some helpful methods/tips on how to go through the process would be appreciated.
feanur
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#2
Jan 31 2016 02:14pm
Working on half of the surface (from θ = 0 to θ = pi/4) :
An infinitesimal shape to compute the surface would be a right triangle, of leg r(θ) and height r(θ).dθ, hence of area r²(θ).dθ/2
So, S = 2. ∫ r²(θ).dθ/2, from 0 to pi/4.
S = ∫ r²(θ).dθ = 2 - pi/2
if I do no mistake.
I guess the estimation of that integral should not be hard.
need2passCalcIII
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#3
Jan 31 2016 02:38pm
Thanks, if I integrated correctly, the answer should be .4292
feanur
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#4
Jan 31 2016 02:42pm
Quote (need2passCalcIII @ Jan 31 2016 09:38pm)
Thanks, if I integrated correctly, the answer should be .4292
Yes, up to 4 decimal places.
need2passCalcIII
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#5
Jan 31 2016 05:43pm
Do you have any idea what the limits would be for this one? Find the area of the region that lies inside the curve r^2=8cos(2θ) and outside the curve r=2. I thought it would be where cos(2θ)=1/2 which means 2θ=pi/3 and 5pi/3, so θ = pi/6 and 5pi/6, but that gives me a wrong answer.
feanur
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#6
Feb 1 2016 02:43pm
Then consider only θ from 0 to pi/6. You'll have to multiply by 4 the following integral :
∫ r²(θ).dθ/2 - pi/3
where pi/3 is the area of the portion of the disk lying between θ = 0 and θ = pi/6.
∫ r²(θ).dθ/2 = ∫ 8.cos(2θ).dθ/2 = 2.sin(pi/3) = √3
...
And finally, S = 4√3 - 4.pi/3 ~ 2.7394
Duckling
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#7
Feb 9 2016 10:52pm
geez makes me feel like my calc 3 class is bad >_>'
we are going over vectors, space geometry and vector value functions right now
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