# Math Help - Determinant of Block Matrix

1. ## Determinant of Block Matrix

Let $M=\begin{pmatrix} A & B \\ O & C\end{pmatrix}$, where A and C are square matrix. Show that det(M)=det(A)det(C)

I have the following idea but i dont have a concrete proof. We can perform elementary row operations on the matrix M. Eventually A and C will be a upper triangular matrix. Hence det(A) and det(C) will just be the product of their diagonal entries. On the other hand, M will also become an upper triangular matrix after the elementary row operations. Therefore, det(M)=product of its diagonal entries=product of diagonal entries of A * product of diagonal entries of C=det(A)det(C).

How do i write a concrete proof using this idea or is there any other method of doing it (e.g. cofactor expansion, etc.)

2. ## Re: Determinant of Block Matrix

Originally Posted by H12504106
Let $M=\begin{pmatrix} A & B \\ O & C\end{pmatrix}$, where A and C are square matrix. Show that det(M)=det(A)det(C)

I have the following idea but i dont have a concrete proof. We can perform elementary row operations on the matrix M. Eventually A and C will be a upper triangular matrix. Hence det(A) and det(C) will just be the product of their diagonal entries. On the other hand, M will also become an upper triangular matrix after the elementary row operations. Therefore, det(M)=product of its diagonal entries=product of diagonal entries of A * product of diagonal entries of C=det(A)det(C).
You can perform elementary row operations on the matrix $M$ but then you will take a matrix $M_1$ with

$det(M_1) \neq det(M)$.

Originally Posted by H12504106
How do i write a concrete proof using this idea or is there any other method of doing it (e.g. cofactor expansion, etc.)
One method is by induction on $n$ - using cofactor expansion - where $n$ is the type of $A$ i.e. $A$ is an $n\times n$ matrix.