更改

[[Image:Animated illustration of inflection point.gif|upright=2.5|thumb|Plot of {{math|''f''(''x'') {{=}} sin(2''x'')}} 从<math>\pi /4 </math>到<math>5 \pi /4 </math>; 该函数二阶导数为{{math|{f}''(x) {{=}} –4sin(2''x'')}}, 和{{mvar|f}}符号相反。曲线为凸时（函数在切线上方）切线颜色为蓝色，曲线为凹时（函数在切线下方）切线颜色为绿色，拐点颜色为红色：0, {{pi}}/2 and {{pi}}]]

[[Image:Animated illustration of inflection point.gif|upright=2.5|thumb|Plot of {{math|''f''(''x'') {{=}} sin(2''x'')}} 从<math>\pi /4 </math>到<math>5 \pi /4 </math>; 该函数二阶导数为<math>{f}''(x) = –4sin(2x)</math>, 和{{mvar|f}}符号相反。曲线为凸时（函数在切线上方）切线颜色为蓝色，曲线为凹时（函数在切线下方）切线颜色为绿色，拐点颜色为红色：0, <math>\pi /2</math> 和 <math>\pi</math>]]

==必要非充分条件==

==必要非充分条件==