数列的通项公式反映了数列{a_n}的第n项与a_n之间的关系。要研究数列的性质(如单调性、有界性、周期性、一些特定的关系等式)都有赖于数列的通项公式,求数列的通项公式是数列问题中的基本问题。策略一观察法例1已知数列1,-2/5,9/(35),-4/(21),5/(33),-(18)/(143),…,求该数列的通项公式。分析观察数列中各项a_n与其序号n之间的 The general formula for a series reflects the relationship between the nth term of a series {a_n} and a_n. To study the nature of the series (such as monotonicity, boundedness, periodicity, some specific relational equations) depends on the general formula of the series. The general formula of the sequence of numbers is the basic problem in the sequence of numbers. Strategy 1 Observation Law 1 Known series 1, -2 / 5,9 / (35), -4 / (21), 5 / (33), - (18) / (143), ..., Item formula Analyze the observed sequence of a_n and its serial number n