Unexpected answers offered by Computer Algebra Systems to school equations

The mathematics curricula of different countries and schools contain different, but certainly significant, amounts of equations. Obviously, the equation types (linear, quadratic and fractional equations, equations that contain an absolute value of an expression, irrational, exponential, logarithmic, trigonometric and literal equations) examined in this paper constitute a large portion of all such equations. The paper focuses on answers offered to different equations by Computer Algebra Systems (here, Axiom, Maxima, Sage, WIRIS and WolframAlpha). Most of the answers are customary for school, but some answers can be somewhat unexpected. The aim of the paper is to classify and map such unexpected answers. 29 phenomena are identified and grouped. The groups are labeled as Input, Form, Unfinished, Domain, Branches, and Automatic. Several phenomena seem to have didactic value for in-depth treatment of certain topics (e.g., equivalence, domain).