[MATLAB基础] 求助！一个小程序总是提示该错误：??? Conversion to double from sym is not possible.

[psh]

下面程序：
clear
clc
len_mem = 10;
mu = 6;
ser_mean = 1/mu;
events = [];
events(2,: ) = exprnd(ser_mean,1,len_mem);
for i=1:len_mem
syms x;
f=ser_mean*exp(ser_mean*x);
events(9,i)=int(x*f,0,events(2,i));
end