Why the multiplication between two matrix is wrong?

Questions
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1

I did the multiplication in paper and the result is different and I made it too in other software and the result is the same that I made in paper

problem2.JPG
problem2.JPG604×185 11.7 KB

2

It would help if you posted your code in <> so we can copy it. what result did you get doing it by hand and what did you get in Mathematica?

3

L = {{1, 0, 0, 0}, {3, 1, 0, 0}, {2, 4, 1, 0}, {5, 2, 3, 1}}
U = {{1, 9, 7, 5}, {0, 1, 8, 5}, {0, 0, 1, 2}, {0, 0, 0, 1}}
L*U
{{1, 0, 0, 0}, {0, 1, 0, 0}, {0, 0, 1, 0}, {0, 0, 0, 1}}

This is the result that give me in the other software

11311770_1592516921017206_1036035597_n.jpg
11311770_1592516921017206_1036035597_n.jpg660×269 20.8 KB

4

Try L.U for the matrix product instead of L*U which does element-wise multiplication.

5

In[35]:= l = {{1, 0, 0, 0}, {3, 1, 0, 0}, {2, 4, 1, 0}, {5, 2, 3, 1}};

In[36]:= u = {{1, 9, 7, 5}, {0, 1, 8, 5}, {0, 0, 1, 2}, {0, 0, 0, 1}};

In[37]:= l .u

Out[37]= {{1, 9, 7, 5}, {3, 28, 29, 20}, {2, 22, 47, 32}, {5, 47, 54,
42}}