Where have I made mistakes in this code

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Lukgaf
Lukgaf 2018-3-22
评论： Walter Roberson 2018-3-22
%prob2.m
% u’’- 4u’ + 4u = exp(x)+ C;
% Subject to u(1) = 0; u(-1) = 0; C=(-4*exp)/(1+(exp)^2);
N = 40;
[D, x]=cheb(N); %%we call this function to get the Derivative matrix D
I = eye(N+1); % I is the identity matrix of size (N+1)
C=(-4*exp(x))/(1+(exp(x).^2)); %the value of the constant
A = D^2 - 4*D + 4*I; f = exp(x)+ C;
B = A(2:N,2:N); %to impose the boundary conditions
ff = f(2:N);
v = B\ff; %solve the BVP.
u = [0;v;0]; xx = -1:0.1:1;
u_exact = exp(xx)-(sinh(1)/sinh(2))*exp(2*xx)+ C/4;
plot(x,u); %plot the numerical solution
xlabel('x')
ylabel('u')
hold on
plot(xx,u_exact,'o'); % plot the exact solution
hold off
I have used the similar code to solve many questions, why is not working for this? Thanks. It shows this eror: Error
in prob2 (line 11)
v = B\ff; %solve the BVP.
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Lukgaf
Lukgaf 2018-3-22
>> prob2
Error using \
Matrix dimensions must agree.
Error in prob2 (line 11)
v = B\ff; %solve the BVP.
Jan
Jan 2018-3-22
编辑：Jan 2018-3-22
@Lukgaf: This is the line, which causes the error. But what is the message, which explains what the problem is?

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Walter Roberson
Walter Roberson 2018-3-22
v = B\ff.'; %solve the BVP.
  2 个评论
Lukgaf
Lukgaf 2018-3-22
Thanks all. But why is the exact soluation has the similar problem.
Matrix dimensions must agree.
Error in prob2 (line 13)
u_exact = exp(xx)-(sinh(1)/sinh(2))* exp(2*xx)+ C/4;
Walter Roberson
Walter Roberson 2018-3-22
You would get that error if xx is a vector or array and C is a vector or array of different size than xx (though starting in R2016b some size combinations can be meaningfully computed as if you had used bsxfun)

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