Saturday, 8 August 2015

calculus - Rewrite double integral in polar coordinates int30int1x/3fdydx



I have the following...

301x/3fdydx



I need to rewrite this in polar coordinates. I graphed the triangle and can see that...



tan(θ)=13



But I do not know how to use this information to rewrite my integral into polar coordinates using π.


Answer



For simplicity, I will assume that f(x,y)=1.




Note that if you are given f(x,y) explicitly, you have to re-express f(x,y) in terms of r and θ.



Using the hint that x=rcosθ as mentioned in the comment section, and y=1,



1=rsinθr=1sinθ



Also,



tan(θ)=13θ=arctan(13)




So we can setup the double integral as the following,



π2arctan(13)1sinθ0r drdθ



Below is a (ugly drawn) picture for a fixed (r,θ).



enter image description here



(Another exercise) Rewrite the double integral in polar coordinates




30x/30 dydx



Let x=3,



3=rcosθr=3cosθ



Also, tan(θ)=13θ=arctan(13)



So we can setup the double integral as the following,




arctan(13)03cosθ0r drdθ



Below is another (ugly drawn) picture for a fixed (r,θ).



enter image description here


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