small decorative article

To the approximation for small angle,the first-order analytical expression of the angular displacement and its rate,and the second-order analytical expression of the length a.

在小摆角近似下,采用迭代方法推导出摆角、摆角变化率的一级近似解析表达式和摆长、摆长变化率的二级近似解析表达式。