In mathematic along with pure mathematic style the solution of practical problems takes big part thinking. Style of thinking is slightly different, notably, so named rational reasoning. Such reasoning allowed digression from exact logic. There are used, for example non-strict definitions, allowed partial replacement in deductive conclusion that based on common sense, analogs, insufficient induction, and physical thinking. Necessity in such reasoning defined in various practical thinking that appears in the solutions in applied problems, such for example, limitedness of time, economical appropriateness then etc. Rational reasoning less schematizes and idealizes reality than deductive conclusion of pure mathematic; therefore, they are more fitting to analyze real facts and situations. Ability to take rational reasoning is important component of general intellectual culture of modern mankind. But its form is important reserve to deepen applied direct of educating mathematic.