Avoiding simple errors like that is what Computer Algebra Systems (CAS) are all about and I've found Maxima to be an extremely powerful CAS program. Maxima is licensed under the GPL, so it has to be ...

So, I set about learning one: let the computers do the math! The package I chose is wxMaxima, a document-based front-end to the Maxima computer-algebra system.

Jinhu Li, Jeffrey S. Racine, Maxima: An Open Source Computer Algebra System, Journal of Applied Econometrics, Vol. 23, No. 4 (Jun., 2008), pp. 515-523 ...