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Without expanding, show that the value of the following determinant is zero: ∣ ∣ ∣ ∣ a + b 2 a + b 3 a + b 2 a + b 3 a + b 4 a + b 4 a + b 5 a + b 6 a + b ∣ ∣ ∣ ∣

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Question

Without expanding, show that the value of the following determinant is zero:

\[\begin{vmatrix}a + b & 2a + b & 3a + b \\ 2a + b & 3a + b & 4a + b \\ 4a + b & 5a + b & 6a + b\end{vmatrix}\]

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Solution

\[ ∆ = \begin{vmatrix}a + b & 2a + b & 3a + b \\ 2a + b & 3a + b & 4a + b \\ 4a + b & 5a + b & 6a + b\end{vmatrix}\]
\[ = \begin{vmatrix}a & a & a \\ 2a & 2a & 2a \\ 4a + b & 5a + b & 6a + b\end{vmatrix} \left[ \text{ Applying } R_1 \to R_2 - R_1  \text{ and }  R_2 \to R_3 - R_2 \right]\]
\[ = 2\begin{vmatrix}a & a & a \\ a & a & a \\ 4a + b & 5a + b & 6a + b\end{vmatrix} = 0\]

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Chapter 5: Determinants - Exercise 6.2 [Page 57]

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R.D. Sharma Mathematics Volume 1 and 2 [English] Class 12
Chapter 5 Determinants
Exercise 6.2 | Q 2.05 | Page 57

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