need help with problem

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dulanga 2019-4-3

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评论： dulanga 2019-4-4

采纳的回答： Matt J

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what is wrong in this code i dont get the correct answer

xleft=0; 
xright=(9*pi)/4;
N=20;
x=linspace(xleft,xright,N);
x=x';
dx=(xright-xleft)/(N-1);
yleft=1;
yright=1;
a=1;
b=-2;
c=-2;
% M is the matrix when multiplied by y gives y'' y' and y
M2=(diag(-2*ones(N,1),0)+diag(ones(N-1,1),-1)+diag(ones(N-1,1),1))/dx^2;
M1=(diag(ones(N-1,1),1)-diag(ones(N-1,1),-1))/(2*dx);
M0=eye(N);
M=a*M2+b*M1+c*M0;
%treating boundaries separate
M(1,:)=[1,zeros(1,N-1)];
M(end,:)=[zeros(1,N-1),1];
k=(sin(x))/((cos(x))+2);
k(1) = yleft; 
k(end) = yright; 
y=M\k

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采纳的回答

Matt J 2019-4-3

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is my code correct i dont get the correct answer

Then by definition, it is not correct...

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Matt J 2019-4-4

编辑：Matt J 2019-4-4

Why is there no M1? The only thing that has changed is b=c=0.

dulanga 2019-4-4

yes so when it goes to M equation since b is zero M1 doesnt matter anymore ?

There is a M1 yes since b*M1 and b is zero M1 holds no impact on M then ?

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Muhammad Ibrahim 2019-4-4

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编辑：Muhammad Ibrahim 2019-4-4

k=(sin(x))/((cos(x))+2);

should be

k=(sin(x))./((cos(x))+2);

That small dot after sin(x) can make a big difference.

Also. read the question carefully, you might be keying the maximum value when it maybe asking minimum value.If you still have problems just ask again in this forum maybe I can help out.