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Previous Year Question (PYQs)
3
The value of $\lambda$ for which the volume of the parallelepiped formed by the vectors
$\vec i+\lambda\vec j+\vec k,\ \vec j+\lambda\vec k,\ \lambda\vec i+\vec k$
is minimum is
Solution
Volume $=|\det|$
$V=|\lambda^3-3\lambda|$
Minimum when derivative $=0$
$\Rightarrow 3\lambda^2-3=0$
$\Rightarrow \lambda=\pm\dfrac1{\sqrt3}$
Minimum positive value at $\lambda=\dfrac1{\sqrt3}$
Answer: $\boxed{\dfrac1{\sqrt3}}$
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