Hullo all. I have just started s job with Chegg, and my 1st assignment has me writign solutions for Stewart's Essential Calculus. I am dealing with the chapter on multiple integration, and need a good open-source software that I can easily use to draw regions (domains) that would require multiple integrals: i.e. circular regions, portions of circles, regions bounded by curves and/or arcs, and at some point 3D pictures.

In most of these cases, I am not working with an exact equation or perhaps need to draw the region bounded by r (radius) between 1 and 2, and the angle theta bounded between pi/4 and 3 pi/4. I am not too terribly familiar with programs like Corel Draw, but the documents I have from this company suggest Corel Draw. So I think I am looking for an open-source free program like Corel Draw or something similar. Any additional suggestions would also be appreciated. I know I can do most, if not all of this using TikZ, but the learning curve is a bit steep, and at the moment I an on a time constraint. Thanks.