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Previous Year Question (PYQs)
2
Let $\vec{a}=2\hat{i}-\hat{j}+\hat{k}$ and $\vec{b}=\lambda\hat{j}+2\hat{k}$, $\lambda\in Z$. Let $\vec{c}=\vec{a}\times\vec{b}$ and $\vec{d}$ be a vector of magnitude $2$ in $yz$-plane. If $|\vec{c}|=\sqrt{53}$, then the maximum possible value of $(\vec{c}\cdot\vec{d})^2$ is:
Solution
$\vec{c}=\vec{a}\times\vec{b} \Rightarrow |\vec{c}|=\sqrt{53}$. Maximum dot when $\vec{d}$ parallel to projection of $\vec{c}$ on yz-plane → $(\vec{c}\cdot\vec{d})_{max}=|\vec{c}|\cdot|\vec{d}|=\sqrt{53}\cdot2$. Square = $4\times53=104$
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