[Math] Using elementary row operations to find determinant 4×4

matrices

I need to find the determinant of this matrix $A$ $$\begin{pmatrix}
-1 & 2& -3& 4\\
5 & 0& 2& -2\\
2& 1& 1& 2\\
0& 0& 3& -2
\end{pmatrix}$$ I've found it using the cofactor method and got:
$$\det(A) = 60$$

however i find it difficult to use elementary row operations to find that – can somebody help?

Best Answer

$$\begin{pmatrix} -1 & 2& -3& 4\\ 5 & 0& 2& -2\\ 2& 1& 1& 2\\ 0& 0& 3& -2 \end{pmatrix}\stackrel{R_2+5R_1\;,\;\;R_3+2R_1}\longrightarrow\begin{pmatrix} -1 & 2& -3& 4\\ 0 & 10& -13& 18\\ 0& 5& -5& 10\\ 0& 0& 3& -2 \end{pmatrix}\stackrel{R_3-\frac12R_2}\longrightarrow$$

$$\begin{pmatrix} -1 & 2& -3& 4\\ 0 & 10& -13& 18\\ 0& 0& 1.5& 1\\ 0& 0& 3& -2 \end{pmatrix}\stackrel{R_4-2R_3}\longrightarrow\begin{pmatrix} -1 & 2& -3& 4\\ 0 & 10& -13& 18\\ 0& 0& 1.5& 1\\ 0& 0& 0& -4 \end{pmatrix}$$

The matrix is now an upper triangular one and its determinant is just the product of its main diagonal's entries...which is certainly $\;60\;$ .