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