Matlab无法解决这个微分方程

Question

我正在模拟BOD，NH4，有机氮，硝酸盐，亚硝酸盐和DO的命运，并在Matlab中遇到了这个错误。 DO的代码不起作用。

我的代码是：

    syms L(x)
    syms DO(x)
    syms ON(x)
    syms NH(x)
    syms NO2(x)
    syms NO3(x)
    syms v
    syms kd
    syms k1
    syms k2
    syms k3
    syms ka
    syms L0
    syms DO0
    syms DOs
    syms ON0
    syms NH0
    syms NO20
    syms NO30
    L(x) = dsolve(diff(L) == -kd*L/v, L(0) == L0)
    ON(x) =dsolve(diff(ON)==-k1*ON/v, ON(0)==ON0)
    NH(x)=dsolve(diff(NH)==-k2*NH/v+k1*ON(x)/v, NH(0)==NH0)
    NO2(x)=dsolve(diff(NO2)==-k3*NO2/v+k2*NH(x)/v, NO2(0)==NO20)
    NO3(x)=dsolve(diff(NO3)==k3*NO2(x)/v, NO3(0)==NO30)
    S= dsolve(diff(DO)==-kd*L(x)/v-3.43*k2*NH(x)/v-1.14*k3*NO2(x)/v+(DOs-DO)*ka/v, 
    DO(0)==DO0)       

错误是：

    Error using mupadengine/feval (line 157)
    MuPAD error: Error: Invalid equation or initial condition. [ode::splitSys]

    Error in dsolve>mupadDsolve (line 325)
    T = feval(symengine,'symobj::dsolve',sys,x,options);

    Error in dsolve (line 186)
    sol = mupadDsolve(args, options);

    Error in Project_Guha_final (line 25)
    S= dsolve(diff(DO)==-kd*L(x)/v-3.43*k2*NH(x)/v-1.14*k3*NO2(x)/v+(DOs-DO)*ka/v,
    DO(0)==DO0) 

我不知道为什么我会遇到这个问题。