- #1
mathman44
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Homework Statement
Use GT to find the area of one petal of the 8-leafed rose given by
[tex]r=17sin(\theta)[/tex]
Recall that the area of a region D enclosed by a curve C can be found by
[tex]A=1/2\int(xdy - ydx)[/tex]
I calculated it using the parametrization
[tex]x=rcos(\theta), y=rcos(\theta)[/tex]
And I found a really long integral, evaluated it from 0 to pi/4, and got the correct answer.
Here is my question: apparently, if x is defined as above, and I find
[tex] dx = -rsin(\theta), dy = rcos(\theta)[/tex], then the integral
[tex]A=1/2\int(xdy - ydx)[/tex] simplifies nicely to [tex]1/2\int(r^2)d\theta[/tex]. Evaluating this integral again from 0 to pi/4 gives the correct answer.
So... why is it that I can pretend "r" is a constant when I'm evalutating dx and dy, when really, r is dependent on theta just as the x and y parametrizations are?