It's probably not a good solution for most people, but here's what I do: I have a TI-89 emulator on my android phone. I used one of those calculators for about

It's probably not a good solution for most people, but here's what I do: I have a TI-89 emulator on my android phone. I used one of those calculators for about six years in high school and college, and I'll never lose the muscle memory of the interface. (I also still own the device, which justifies using the emulator ethically, if not strictly legally.) I believe it's actually faster for me than, say, sympy, which I also use in other contexts. Also faster than using pencil and paper, for sufficiently large problems. If you're not familiar, it uses a CAS called derive which handles algebraic simplification (including trigonometry), differentiation and integration. Oh, and a function called "rref" (reduced row echelon form; familiar to matlab users), which solves a linear system (including symbolically), with absolutely minimal overhead in the "CLI". Now that I say that, I want an actual CLI tool called rref that works in the same way.