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Previous Year Question (PYQs)
3
The vectors $\vec{a},\vec{b},\vec{c}$ are equal in length and taken pairwise make equal angles.
If $\vec{a}=\hat{i}+\hat{j}$, $\vec{b}=\hat{j}+\hat{k}$ and $\vec{c}$ makes an obtuse angle with $\hat{i}$, then $\vec{c}$ is equal to
Solution
Equal lengths and equal pairwise angles ⇒ vectors form a symmetric set.
Dot-products must satisfy:
$\vec{a}\cdot\vec{b}=\vec{b}\cdot\vec{c}=\vec{c}\cdot\vec{a}$
Also $\vec{c}$ must make obtuse angle with $\hat{i}$ ⇒ its $i$-component < 0.
Solving gives:
$\vec{c}=\frac{1}{3}\hat{i}+\frac{4}{3}\hat{j}-\frac{1}{3}\hat{k}$
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