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JEE Advanced 2021 · Determinants

For any 3×33 \times 3 matrix MM, let ∣M∣|M| denote the determinant of MM. Let II be the 3×33 \times 3 identity matrix. Let EE and FF be two 3×33 \times 3 matrices such that (I−EF)(I - EF) is invertible. If G=(I−EF)−1G = (I - EF)^{-1}, then which of the following statements is (are) TRUE?

Answers: A. ∣FE∣=∣I−FE∣∣FGE∣|FE| = |I - FE||FGE| · B. (I−FE)(I+FGE)=I(I - FE)(I + FGE) = I · C. EFG=GEFEFG = GEF

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For any 3 × 3 matrix M, let |M| denote the determinant of M. Let I be the 3 × 3 identity matrix. Let E and F be two 3 × 3 matrices such that (I - EF) is invertible. If G = (I - EF)⁻¹, then which of the following statements is (are) TRUE? Given: G=(I−EF)−1,I−EF invertible,∣M∣=det⁡M\text{Given: } G = (I - EF)^{-1},\quad I - EF \text{ invertible},\quad |M| = \det M To find: which of (1)–(4) are TRUE\text{To find: which of (1)--(4) are TRUE} 1. from G=(I−EF)−1: EFG, GEF1.\ \text{from } G = (I-EF)^{-1}:\ EFG,\ GEF 2. determinant identity for (1)2.\ \text{determinant identity for (1)} 3. expand the products for (2), (4)3.\ \text{expand the products for (2), (4)} (I−EF)G=IandG(I−EF)=I(I - EF)G = I \quad \text{and} \quad G(I - EF) = I (I−EF)G=I⇒G−EFG=I⇒EFG=G−I(I-EF)G = I \Rightarrow G - EFG = I \Rightarrow EFG = G - I G(I−EF)=I⇒G−GEF=I⇒GEF=G−IG(I-EF) = I \Rightarrow G - GEF = I \Rightarrow GEF = G - I ⇒EFG=GEF⇒ (3) TRUE\Rightarrow EFG = GEF \quad\Rightarrow\ \text{(3) TRUE} ∣I−EF∣=∣I−FE∣|I - EF| = |I - FE| ∣G∣=1∣I−EF∣=1∣I−FE∣|G| = \dfrac{1}{|I - EF|} = \dfrac{1}{|I - FE|} ∣FGE∣=∣F∣ ∣G∣ ∣E∣=∣FE∣ ∣G∣=∣FE∣∣I−FE∣|FGE| = |F|\,|G|\,|E| = |FE|\,|G| = \dfrac{|FE|}{|I - FE|} ⇒ ∣FE∣=∣I−FE∣ ∣FGE∣⇒ (1) TRUE\Rightarrow\ |FE| = |I - FE|\,|FGE| \quad\Rightarrow\ \text{(1) TRUE} FE FGE=F(EFG)E=F(G−I)E=FGE−FEFE\,FGE = F(EFG)E = F(G-I)E = FGE - FE (I−FE)(I+FGE)=I+FGE−FE−FE FGE(I - FE)(I + FGE) = I + FGE - FE - FE\,FGE =I+FGE−FE−(FGE−FE)=I⇒ (2) TRUE= I + FGE - FE - (FGE - FE) = I \quad\Rightarrow\ \text{(2) TRUE} (I−FE)(I−FGE)=I−FGE−FE+FE FGE(I - FE)(I - FGE) = I - FGE - FE + FE\,FGE =I−FGE−FE+(FGE−FE)=I−2FE≠I= I - FGE - FE + (FGE - FE) = I - 2FE \neq I ⇒ (4) FALSE\Rightarrow\ \text{(4) FALSE} True: (1), (2), (3)\boxed{\text{True: } (1),\ (2),\ (3)}

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