Q.Evaluate $\Delta = \begin{vmatrix} 1 & a & b+c \\ 1 & b & a+c \\ 1 & c & a+b \end{vmatrix}$.

Minor of a Matrix Element – From Intuition to Precision

Imagine you have a large matrix — say a $3 \times 3$ grid of numbers. You pick one specific element, like the number in the second row and third column. Now ask: if I wanted to ignore that element and everything in its row and column, what smaller matrix remains? That smaller matrix is the key.

The minor of an element is simply the determinant of that leftover submatrix. It captures the "influence" of that element when you strip away its entire row and column.

The Intuition First

Think of a matrix as a system of equations. Each row is an equation, each column a variable. When you focus on one element, you are asking: what happens to the rest of the system if I remove the equation and the variable that this element belongs to? The minor tells you the "size" (determinant) of that reduced system.

For a $2 \times 2$ matrix, the minor of any element is just the single number diagonally opposite it — because removing one row and one column leaves a $1 \times 1$ matrix, whose determinant is that number itself.

For a $3 \times 3$ matrix, the minor of an element is the determinant of a $2 \times 2$ matrix formed by the four numbers that are not in the same row or column as the chosen element.

The Precise Statement

Let $A$ be an $n \times n$ square matrix. Let $a_{ij}$ be the element in the $i$-th row and $j$-th column.

$M_{ij}$ is called the minor of the element $a_{ij}$.

A Concrete Example

Take the matrix:

Find the minor of the element $a_{23} = 8$ (row 2, column 3).

Delete row 2 and column 3. What remains?

The minor is the determinant of this $2 \times 2$ matrix:

So the minor of $8$ is $-6$.

Why Minors Matter

Minors are the building blocks of cofactors, which in turn are used to compute determinants of large matrices (Laplace expansion) and to find inverses. Every time you expand a determinant along a row or column, you are summing products of elements and their minors (with appropriate signs).

A Common Mistake to Avoid

Students often confuse the minor with the element itself or with the submatrix. The submatrix is the $2 \times 2$ block of numbers; the minor is the determinant of that block. They are not the same thing.

Quick Check

For the same matrix $A$ above, find $M_{12}$ (minor of $a_{12}=4$).

Delete row 1 and column 2:

Notice that $M_{12} = M_{23}$ in this particular matrix — that is a coincidence, not a rule.

Final takeaway: The minor of an element is the determinant of the matrix you get by crossing out that element's row and column. It is a single number that measures the "weight" of that element in the larger matrix's structure.