You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: fseriessym.m
    path of file: ./repos/Algorithms_MathModels/《基于MATLAB的高等数学问题求解》 随书附带源程序/CH09
    
    the code of the file until where you have to start completion: function [A,B,F]=fseriessym(f,x,n,a,b)
%FSERIESSYM   傅里叶级数的符号求解
% [A,B,F]=FSERIESSYM(FUN,X,N)  将函数FUN在区间[-pi,pi]上展成N阶傅里叶级数
% [A,B,F]=FSERIESSYM(FUN,X,N,ALPHA,BETA)  将函数FUN在指定区间上展成N阶傅里叶级数

    
function [A,B,F]=fseriessym(f,x,n,a,b)
%FSERIESSYM   傅里叶级数的符号求解
% [A,B,F]=FSERIESSYM(FUN,X,N)  将函数FUN在区间[-pi,pi]上展成N阶傅里叶级数
% [A,B,F]=FSERIESSYM(FUN,X,N,ALPHA,BETA)  将函数FUN在指定区间上展成N阶傅里叶级数
%
% 输入参数：
%     ---FUN：给定的待展开函数
%     ---X：自变量
%     ---N：展开项数
%     ---ALPHA,BETA：级数展开区间，默认值为[-pi,pi]
% 输出参数：
%     ---A,B：傅里叶系数向量
%     ---F：函数的傅里叶展开式
%
% See also int

if nargin==3
    a=-pi;b=pi; 
end