Finding LU factorization and one number is off

Question

I'm trying to find LU of this 2x2 matrix, but when I check my work, the bottom right number is 31 instead of 1. A is the matrix at the top. What am I doing incorrectly?

Answer 1

Considere $A=\begin{pmatrix} 2 & 8 \\ 4 & 1 \\ \end{pmatrix} $ Then we do $A(-2) _{1,2}$ the elementary operation. Then we have Considere $U= \begin{pmatrix} 2 & 8 \\ 0 & - 15\\ \end{pmatrix}$ THEN $L=\begin{pmatrix} 1 & 0 \\ 2 & 1\\ \end{pmatrix} $ And $$LU= \begin{pmatrix} 1& 0 \\ 2& 1\\ \end{pmatrix} \cdot \begin{pmatrix} 2 & 8 \\ 0 & - 15\\ \end{pmatrix} =A$$