matlab solve解不出来

Question

材料力学里面的计算题
clear

close all
clc

syms M L Fd E I z1 z2 C1 C2 C3 C4 C5 C6 C7 C8

v1(z1) = (C1*z1^3/6 + C2*z1^2/2 + C3*z1 + C4)/(E*I);
v2(z2) = (C5*z2^3/6 + C6*z2^2/2 + C7*z2 + C8)/(E*I);

Dv1(z1) = diff(v1,z1,1);
D2v1(z1) = diff(v1,z1,2);
D3v1(z1) = diff(v1,z1,3);
Dv2(z2) = diff(v2,z2,1);
D2v2(z2) = diff(v2,z2,2);
D3v2(z2) = diff(v2,z2,3);

eq = {'E*I*D2v1(0) = 0',...
'Dv2(L) = 0',...
'v2(L) = 0',...
'v1(0) = 0',...
'Dv1(L) = Dv2(0)',...
'v1(L) = v2(0)',...
'D3v1(L) = D3v2(0)',...
'-E*I*D2v1(L) = -E*I*D2v2(0) + M'};
Const = {'C1','C2','C3','C4','C5','C6','C7','C8'};
C = solve(eq{:},Const{:});
Const = [C.C1 C.C2 C.C3 C.C4 C.C5 C.C6 C.C7 C.C8].';
pretty(Const);
以上代码解不出来，如果将eq里包含的函数方程解出再计算，例如用如下代码则可行
clear
close all
clc

syms M L Fd z1 z2 E I C1 C2 C3 C4 C5 C6 C7 C8
eq = {'C2 = 0',...
'((C5*L^2)/2 + C6*L + C7)/(E*I) = 0',...
'((C5*L^3)/6 + (C6*L^2)/2 + C7*L + C8)/(E*I) = 0',...
'C4/(E*I) = 0',...
'((C1*L^2)/2 + C2*L + C3)/(E*I) = C7/(E*I)',...
'((C1*L^3)/6 + (C2*L^2)/2 + C3*L + C4)/(E*I) = C8/(E*I)',...
'C1/(E*I) = C5/(E*I)',...
'- C2 - C1*L = -C6 + M'};

Const = {'C1','C2','C3','C4','C5','C6','C7','C8'};

C = solve(eq{:},Const{:});
Const = [C.C1 C.C2 C.C3 C.C4 C.C5 C.C6 C.C7 C.C8].';
pretty(Const);

请问是什么原因导致的？

Answer 1

原因一：
用这样的表达式
v1(z1) = (C1*z1^3/6 + C2*z1^2/2 + C3*z1 + C4)/(E*I);
来描述v1关于z1的函数是不正确的。
应该直接写就可以了
v1 = (C1*z1^3/6 + C2*z1^2/2 + C3*z1 + C4)/(E*I);
原因二：
这样的表达式'E*I*D2v1(0) = 0'也有问题，这里不存在初值的问题，v1(0)