设给定了一个数列{u_n}: u_1,u_2,u_3,…,u_n,…求它的前n项的和 S_n=sum from k=1 to n u_k=u_1+u_2+u_3+…+u_k+…+u_n 通常也把它叫做求数列sum from k=1 to n u_k的和,这里u_k表示这个数列的通项。如果数列的通项u_k是三角函数或者反三角函数的表达式,例如 Let’s give a sequence {u_n}: u_1,u_2,u_3,...,u_n,... Find the sum of the first n terms and S_n=sum from k=1 to n u_k=u_1+u_2+u_3+...+u_k+...+ U_n is also commonly referred to as the sum of the sum columns sum from k=1 to n u_k, where u_k denotes the general term of this sequence. If the general term u_k of the sequence is an expression of a trigonometric function or an inverse trigonometric function, eg