Aspire Faculty ID #18101 · Topic: JEE Main 2026 (28 January Morning Shift) · Just now
JEE Main 2026 (28 January Morning Shift)

Let $ A, B $ and $ C $ be three $ 2 \times 2 $ matrices with real entries such that $ B = (I + A)^{-1} $ and $ A + C = I $. If $ BC = \left[ \matrix{ 1 & -5 \cr -1 & 2 } \right] $ and $ CB \left[ \matrix{ x_1 \cr x_2 } \right] = \left[ \matrix{ 12 \cr -6 } \right] $, then $ x_1 + x_2 $ is

Solution

$ B = (I + A)^{-1},; A + C = I $

$ \Rightarrow B(I + A) = (I + A)B = I $

$ \Rightarrow B + BA = B + AB = I $

$ \Rightarrow B + B(I - C) = B + (I - C)B $

$ \Rightarrow 2B - BC = 2B - CB $

$ \Rightarrow BC = CB $

$ \therefore CB \left[ \matrix{ x_1 \cr x_2 } \right] = \left[ \matrix{ 1 & -5 \cr -1 & 2 } \right] \left[ \matrix{ x_1 \cr x_2 } \right] = \left[ \matrix{ 12 \cr -6 } \right] $

$ \Rightarrow \left[ \matrix{ x_1 \cr x_2 } \right] = \left[ \matrix{ 1 & -5 \cr -1 & 2 } \right]^{-1} \left[ \matrix{ 12 \cr -6 } \right] $

$ = \frac{1}{-3} \left[ \matrix{ 2 & 5 \cr 1 & 1 } \right] \left[ \matrix{ 12 \cr -6 } \right] $

$ = \frac{1}{-3} \left[ \matrix{ 24 - 30 \cr 12 - 6 } \right] = \left[ \matrix{ 2 \cr -2 } \right] $

$ \therefore x_1 + x_2 = 0 $

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