关于solve函数的一个问题

[yxhhit]

新手求助，现编了一个循环程序，里面用到solve函数，但循环只进行了一步，就提示出错！??? Error using ==> solve at 162
Unable to find closed form solution 。先将程序列于下方，请高人指教！谢谢
主程序
clc
clear

CDF_original_Up = normcdf(11,5,3);
CDF_original_Down = normcdf(-1,5,3);
Down_standard = -2;
Up_standard = 2;

for i=1:4
miu_standard = 0;
theda_standard = double(fun_norm(Down_standard,Up_standard));

Up_standard = norminv(0.9772,miu_standard,theda_standard);
Down_standard = norminv(0.0228,miu_standard,theda_standard);
end
子程序
function y= fun_norm(a,b)
syms t theda
y = solve(int(exp(-t^2/(2*theda^2))/(theda*(2*pi)^0.5),t,a,b)-1);