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Computer algebra systems, such as Mathematica, Maple, SageMath, Matlab's Symbolic Math Toolbox, etc. are very useful for the following tasks (ranked in importance):
In terms of alternatives to the above tasks, there is no real alternative to item 1 in terms of conventional software you will use. Items ranked 2-3 all have alternatives, e.g. Python, Matlab, C++, etc.
I will do some simple 2D, 3D, and contour plotting.
I will do an example showing quadrature using Mathematica. This will look at the SIAM 100-digit challenge problem 1.