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I am trying to solve a system of first-order differential equations of the following form: $$\frac{dr}{dt}=f(r,\theta), \quad\frac{d\theta}{dt}=g(r,\theta)$$ I am using desolve_system_rk4 which gives me a list of triples $(t,r,\theta)$. Now I need to plot the trajectory in the xy-plane. For this, I require to obtain the x and y coordinate using $$x=r\cos\theta, \quad y=r\sin\theta$$ and then obtain the xy-plot. How can I tabulate the pair $(r\cos\theta,r\sin\theta)$ from $(t,r,\theta)$?