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+3 votes
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If the system of equations

\(2 x+3 y-z=5\)

\(x+\alpha y+3 z=-4\)

\(3 x-y+\beta z=7\)

has infinitely many solutions, then \(13 \alpha \beta\) is equal to

(1) 1110

(2) 1120

(3) 1210

(4) 1220

1 Answer

+2 votes
by (50.3k points)
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Best answer

Correct option is (2) 1120

Using family of planes

\(2 \mathrm{x}+3 \mathrm{y}-\mathrm{z}-5=\mathrm{k}_{1}(\mathrm{x}+\alpha \mathrm{y}+3 \mathrm{z}+4)+\mathrm{k}_{2}(3 \mathrm{x}-\mathrm{y}+\beta \mathrm{z}-7)\)

\(2=\mathrm{k}_{1}+3 \mathrm{k}_{2}, 3=\mathrm{k}_{1} \alpha-\mathrm{k}_{2},-1=3 \mathrm{k}_{1}+\beta \mathrm{k}_{2},-5=4 \mathrm{k}_{1}-7 \mathrm{k}_{2}\)

On solving we get

\(k_{2}=\frac{13}{19}, k_{1}=\frac{-1}{19}, \alpha=-70, \beta=\frac{-16}{13}\)

\(13 \alpha \beta=13(-70)\left(\frac{-16}{13}\right)\)

\(=1120\)

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