Answer
See explanation
Work Step by Step
$A=L U-$ given
Then, first column of $A$ is matrix $L$ multiplied with first column of $U$.
In other words, $i$ -th element in first column of $A$ is $i$ -th row of $L$ multiplied with first column of $U$.
First column of $U$ has all zeros except (possibly) first entry, so only first element of each row of $L$ "survives" the multiplication with first column of $U$
In conclusion, first column of $A$ is first column of $L$ multiplied with first entry in first column of $U$.
Similarly, second column of $U$ has only first two nonzero entries, so second column of $A$ is a linear combination of first and second columns of $L$.
