Let a→, b→ and c→ be three non - coplanar unit vectors such that the angle between every pair of them is π3. If a→ × b→+ b→ × c→=pa→+qb→+rc→ , where p, q and r are scalars, then the value of p2+2q2+r2q2 is _______

JEE Advanced · Mathematics · 30. Vector Algebra

Let $\vec{a}, \vec{b}$ and $\vec{c}$ be three non - coplanar unit vectors such that the angle between every pair of them is $\frac{π}{3} .$ If $\vec{a} \times \vec{b} + \vec{b} \times \vec{c} = p \vec{a} + q \vec{b} + r \vec{c}$ , where p, q and r are scalars, then the value of $\frac{p^{2} + 2 q^{2} + r^{2}}{q^{2}}$ is _______

A 6
B 4
C 2
D 12

Verified Solution

Answer & Solution

Correct Answer

(B) 4

Step-by-step Solution

Detailed explanation

|\vec{a}| = |\vec{b}| = |\vec{c}| = 1

\vec{a} \times \vec{b} + \vec{b} \times \vec{c} = p \vec{a} + q \vec{b} + r \vec{c}

\vec{a} . (\vec{b} \times \vec{c}) = p + q (\vec{a} . \vec{b}) + r (\vec{a} . \vec{c})

And

[\vec{a} \vec{b} \vec{c}] = \frac{1}{\sqrt{2}}

p + \frac{q}{2} + \frac{r}{2} = [\vec{a} \vec{b} \vec{c}]

.......(i)

\frac{p}{2} + q + \frac{r}{2} = 0

......(ii)

\frac{p}{2} + \frac{q}{2} + r = [\vec{a} \vec{b} \vec{c}]

......(iii)

\Rightarrow

p = r = - q

\frac{p^{2} + 2 q^{2} + r^{2}}{q^{2}} = 4

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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