The value of determinant \(\left| {\begin{array}{*{20}{c}} 1&a&{b + c}\\ 1&b&{c + a}\\ 1&c&{a + b} \end{array}} \right|\)

Concept:

Properties of Determinant of a Matrix:

• If each entry in any row or column of a determinant is 0, then the value of the determinant is zero.
• For any square matrix say A, |A| = |AT|.
• If we interchange any two rows (columns) of a matrix, the determinant is multiplied by -1.
• If any two rows (columns) of a matrix are same then the value of the determinant is zero.

Calculation:

\(\left| {\begin{array}{*{20}{c}} 1&a&{b + c}\\ 1&b&{c + a}\\ 1&c&{a + b} \end{array}} \right|\)

Apply C2 → C2 + C3 

\( = \left| {\begin{array}{*{20}{c}} 1&{a + b + c}&{b + c}\\ 1&{a + b + c}&{c + a}\\ 1&{a + b + c}&{a + b} \end{array}} \right|\)

Taking common (a + b + c) from column 2, we get

\(= \left( {a + b + c} \right)\left| {\begin{array}{*{20}{c}} 1&1&{b + c}\\ 1&1&{c + a}\\ 1&1&{a + b} \end{array}} \right|\)

As we can see that the first and the second column of the given matrix are equal. 

We know that, if any two rows (columns) of a matrix are same then the value of the determinant is zero.

∴ \(\left| {\begin{array}{*{20}{c}} 1&a&{b + c}\\ 1&b&{c + a}\\ 1&c&{a + b} \end{array}} \right|\)= 0