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Q. If $\vec{ a }$ and $\vec{ b }$ are perpendicular, then $\vec{ a } \times(\vec{ a } \times(\vec{ a } \times(\vec{ a } \times \vec{ b })))$ is equal to

JEE MainJEE Main 2021Vector Algebra

Solution:

$\vec{ a } \cdot \vec{ b }=0$
$\vec{ a } \times(\vec{ a } \times \vec{ b })=(\vec{ a } \cdot \vec{ b }) \vec{ a }-(\vec{ a } \cdot \vec{ a }) \vec{ b }=-|\vec{ a }|^{2} \vec{ b }$
Now $\vec{ a } \times\left(\vec{ a } \times\left(-|\vec{ a }|^{2} \vec{ b }\right)\right)$
$=-|\vec{ a }|^{2}(\vec{ a } \times(\vec{ a } \times \vec{ b }))$
$=-|\vec{ a }|^{2}\left(-|\vec{ a }|^{2} \vec{ b }\right)=|\vec{ a }|^{4} \,\vec{ b }$