求导得出来三个表达式，有三个变量，想用mathematica联立起来求解，要求的是PiB/R/Pis，想先将PiB和R作为参数，代入Pis里先把Pis求出来，但总是求解不出来，代码如下：
Z = 2000; F = 12000000; cr1 = 38; cr2 = 18; cs = 14;
\[Tau]1 = 0.71; \[Tau]2 = 0.9; fa = 320; fk = 1.6; fv = 26;
fe = 1000000; fp = 24; fs = 12; \[Lambda] = 0.8; \[Theta] = 3.8;
S = 100000; t = 10; r = 0.05; \[Beta] = 1.8; g = 20;
wv = 52; wc = 20;
p = r/(1 - (1 + r)^(-t));
T = \[Lambda] fa - \[Lambda] fv - fp - 0.5 \[Lambda] fs;
X = \[Lambda] fa + \[Lambda] cr2 + 0.5 \[Lambda] \[Tau]2 cs - cr1 -
0.5 \[Tau]1 cs;
Yis = Exp[\[Theta] (Pis - wc)]/(Exp[\[Theta] (Pis - wc)] +
Exp[\[Theta] (PiB + g - wv)]);
YiB = Exp[\[Theta] (PiB + g - wv)]/(Exp[\[Theta] (Pis - wc)] +
Exp[\[Theta] (PiB + g - wv)]);
PiB = X - (1 +
ProductLog[
Exp[-1 + (X + g - wv ) \[Theta] - (Pis -
wc) \[Theta]]])/\[Theta];
R = ((2 fe Z p)/(Yis S fk \[Beta] \[Pi]))^(1/3);
Pis = T -
fk \[Beta] R - (1 +
ProductLog[
Exp[(T - wc) \[Theta] - 1 -
fk R \[Beta] \[Theta] - (PiB + g - wv) \[Theta]]])/\[Theta]
图片可能看着清楚点：
实在不知道这个方程应该怎么解，求大神们帮帮忙，万分感谢！

你觉得为什么执行
a = b + 1
b = 2 a - 3
的时候会出错？