Inequalities are considered among the most useful tools of investigation in pure and applied mathematics; yet their didactical aspects have not received much attention in mathematics education research until recently. An important aspect of teaching problem solving at the secondary level deals with the notion of equivalence of algebraic transformations used in replacing inequalities by equations. This paper shows that the appropriate use of computer graphing software has the potential to avoid errors and overcome misconceptions associated with the notion of equivalence in solving inequalities. It demonstrates how mathematical visualization provides learners with a conceptual insight into the sources of errors typical for the secondary mathematics classroom.