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I am having difficulty understanding the following:

[itex]\int^{2π}_{0}(x+y)\,dθ[/itex] = [itex]\int^{2π}_{0} 2a\,dθ[/itex] = [itex]\textbf{4}πa[/itex]

where x and y are the generator lines of an elipse, a is the semimajor axis and θ is the angle formed by x and the major axis.

I understand that x+y = 2a. However I don't understand how to integrate with respect to θ without expressing (x+y) or 2a in terms of θ.

Any help would be much appreciated.

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# Integrating the generator lines of an elliptical orbit

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