In[352]:= Clear["`*"]
In[353]:= y1 = (Exp[a1]*(a1 T theta - b - k*x1)^(1/2*theta^2)*(ax T theta - b -
k*x)^(1/2*theta^2))/(Exp[
a0]*(ax T theta - k*x1)^(1/2*theta^2)*(a0 T theta - k*x)^(1/2*
theta^2)) == Exp[UR]
Out[353]= E^(-a0 + a1) (a0 T theta - k x)^(-(theta^2/2)) (-b + ax T theta - k x)^(
theta^2/2) (-b + a1 T theta - k x1)^(theta^2/
2) (ax T theta - k x1)^(-(theta^2/2)) == E^UR
In[354]:= y2 =
x == NU + NH + NM
Out[354]= x == NH + NM + NU
In[355]:= y3 =
ax == (k*x*(1 + Sqrt[q]))/((1 - Sqrt[q] - b)*T)
Out[355]= ax == (k (1 + Sqrt[q]) x)/((1 - b - Sqrt[q]) T)
In[356]:= y4 =
NH == (NM + NH)*(a1 - ax)/(a1 - a0)
Out[356]= NH == ((a1 - ax) (NH + NM))/(-a0 + a1)
In[357]:=
In[358]:= y5 =
1/2*k*x^2 + NH*b == (NU + NH)*G + NM*q*G
Out[358]= b NH + (k x^2)/2 == G (NH + NU) + G NM q
In[359]:= y6 =
G == (-b -
k*x (1 + (ax - a0)/(a1 - ax)))/((-2*k)/(a*theta*T -
k*x)*(1 + (ax - a0)/(a1 - ax))*(q*NM + NU + NH) -
q*(ax - a0)/(a1 - ax) - 1)
Out[359]= G == (-b - (1 + (-a0 + ax)/(a1 - ax)) k x)/(-1 - ((-a0 + ax) q)/(a1 - ax) - (
2 (1 + (-a0 + ax)/(a1 - ax)) k (NH + NU + NM q))/(a T theta - k x))
In[360]:= Solve[{y1, y2, y3, y4, y5, y6}, {x, b, NH, NM, G, ax}]
跑得整一个巨慢无比，不知道有没有更快的方法
{x, b, NH, NM, G, ax}是因变量，其余全是自变量

过于复杂的符号参数是有可能严重拖慢方程求解的，参看
tieba.baidu.com/p/5005284445