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. Fie \( V \) un spaţiu vectorial de dimensiune 3 peste un corp \( K \) şi \( B=\left\{v_{1}, v_{2}, v_{3}\right\} \) o bază în \( V \). Fie \( B^{*}=\left\{v^{1}, v^{2}, v^{3}\right\} \) baza duală, \( x=5 v_{1}+v_{2}-4 v_{3}, y=v_{1}+6 v_{3} \) şi forma liniară \( F=3 v^{1}+v^{2}+v^{3} \). Să se calculeze \( F(2 x-y) \)

Ask by Rowe Elliott. in Romania
Jan 22,2025

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Answer

\( F(2x - y) = 15 \)

Solution

Pentru a calcula \( F(2x - y) \), vom urma pașii detaliat mai jos: 1. **Definirea vectorilor și a formei liniare:** - \( x = 5v_1 + v_2 - 4v_3 \) - \( y = v_1 + 6v_3 \) - \( F = 3v^{1} + v^{2} + v^{3} \) 2. **Calcularea lui \( 2x - y \):** \[ 2x = 2(5v_1 + v_2 - 4v_3) = 10v_1 + 2v_2 - 8v_3 \] \[ 2x - y = (10v_1 + 2v_2 - 8v_3) - (v_1 + 6v_3) = 9v_1 + 2v_2 - 14v_3 \] 3. **Aplicarea formei liniare \( F \) asupra vectorului \( 2x - y \):** \[ F(2x - y) = 3v^{1}(9v_1 + 2v_2 - 14v_3) + v^{2}(9v_1 + 2v_2 - 14v_3) + v^{3}(9v_1 + 2v_2 - 14v_3) \] Folosind proprietatea bazei duale \( v^{i}(v_j) = \delta^i_j \), unde \( \delta^i_j \) este delta lui Kronecker: \[ v^{1}(9v_1 + 2v_2 - 14v_3) = 9, \quad v^{2}(9v_1 + 2v_2 - 14v_3) = 2, \quad v^{3}(9v_1 + 2v_2 - 14v_3) = -14 \] Astfel: \[ F(2x - y) = 3 \cdot 9 + 2 \cdot 1 + (-14) \cdot 1 = 27 + 2 - 14 = 15 \] **Răspuns:** \( F(2x - y) = 15 \)

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Pentru a calcula \( F(2x - y) \), trebuie mai întâi să determinăm expresia lui \( 2x - y \). Avem: \[ x = 5v_{1} + v_{2} - 4v_{3}, \] deci: \[ 2x = 2(5v_{1} + v_{2} - 4v_{3}) = 10v_{1} + 2v_{2} - 8v_{3}. \] Acum, să scriem \( y \): \[ y = v_{1} + 6v_{3}. \] Acum putem calcula \( 2x - y \): \[ 2x - y = (10v_{1} + 2v_{2} - 8v_{3}) - (v_{1} + 6v_{3}) = 10v_{1} - v_{1} + 2v_{2} - 8v_{3} - 6v_{3} = 9v_{1} + 2v_{2} - 14v_{3}. \] Acum, aplicăm forma liniară \( F \) asupra acestui vector: \[ F = 3v^{1} + v^{2} + v^{3}. \] Deci trebuie să calculăm: \[ F(2x - y) = F(9v_{1} + 2v_{2} - 14v_{3}) = 9F(v_{1}) + 2F(v_{2}) - 14F(v_{3}). \] Deoarece \( F(v^{1}) = 3 \), \( F(v^{2}) = 1 \) și \( F(v^{3}) = 1 \), avem: \[ F(v_{1}) = 3, \quad F(v_{2}) = 1, \quad F(v_{3}) = 1. \] Astfel: \[ F(2x - y) = 9(3) + 2(1) - 14(1) = 27 + 2 - 14 = 27 + 2 - 14 = 15. \] Deci, \( F(2x - y) = 15 \).

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