Mathematics · Vector Algebra

JEE Main 2026 — 24 January, Morning Shift — Question 3

Let the lines L1:r⃗=i^+2j^+3k^+λ(2i^+3j^+4k^)L_{1}: \vec{r}=\hat{i}+2 \hat{j}+3 \hat{k}+\lambda(2 \hat{i}+3 \hat{j}+4 \hat{k}), λ∈R\lambda \in \mathbb{R} and L2:r⃗=(4i^+j^)+μ(5i^+2j^+k^),μ∈RL_{2}: \vec{r}=(4 \hat{i}+\hat{j})+\mu(5 \hat{i}+2 \hat{j}+\hat{k}), \mu \in \mathbb{R}, intersect at the point R . Let P and Q be the points lying on lines L1\mathrm{L}_{1} and L2\mathrm{L}_{2}, respectively, such that ∣PR→∣=29|\overrightarrow{\mathrm{PR}}|=\sqrt{29} and ∣PQ→∣=473|\overrightarrow{\mathrm{PQ}}|=\sqrt{\frac{47}{3}}. If the point P lies in the first octant, then 27(QR)227(\mathrm{QR})^{2} is equal to

  1. Option A:

    340340

  2. Option B:

    360360

    Correct
  3. Option C:

    320320

  4. Option D:

    348348

Answer: B

Step-by-step solution

For POI 2λ+1=5μ+4;3λ+2=2μ+1;4λ+3=μ\begin{aligned} & 2 \lambda+1=5 \mu+4 ; 3 \lambda+2=2 \mu+1 ; 4 \lambda+3=\mu & \end{aligned}

⇒λ=μ=−1\Rightarrow \lambda=\mu=-1

R(−1,−1,−1)P(2λ+1,3λ+2,4λ+3)\mathrm{R}(-1,-1,-1) \quad \mathrm{P}(2 \lambda+1,3 \lambda+2,4 \lambda+3)

PR2=29⇒(2λ+2)2+(3λ+3)2+(4λ+4)2=29\mathrm{PR}^{2}=29 \Rightarrow(2 \lambda+2)^{2}+(3 \lambda+3)^{2}+(4 \lambda+4)^{2}=29

⇒λ=0 or λ=−2 (Reject) \Rightarrow \lambda=0 \text { or } \lambda=-2 \text { (Reject) }

⇒P(1,2,3)\Rightarrow \mathrm{P}(1,2,3)

Q(5μ+4,2μ+1,μ)\mathrm{Q}(5 \mu+4,2 \mu+1, \mu)

∣PQ∣=473⇒PQ2=473|\mathrm{PQ}|=\sqrt{\frac{47}{3}} \Rightarrow \mathrm{PQ}^{2}=\frac{47}{3}

⇒(5μ+3)2+(2μ−1)2+(μ−3)2=473\Rightarrow(5 \mu+3)^{2}+(2 \mu-1)^{2}+(\mu-3)^{2}=\frac{47}{3}

⇒μ=−13\Rightarrow \mu=-\frac{1}{3}

Q=(73,13,−13)\mathrm{Q}=\left(\frac{7}{3}, \frac{1}{3},-\frac{1}{3}\right)

(QR)2=(73+1)2+(13+1)2+(−13+1)2(\mathrm{QR})^{2}=\left(\frac{7}{3}+1\right)^{2}+\left(\frac{1}{3}+1\right)^{2}+\left(-\frac{1}{3}+1\right)^{2}

=100+16+49=1209=\frac{100+16+4}{9}=\frac{120}{9}

⇒27×(QR)2=27×1209=360\Rightarrow 27 \times(\mathrm{QR})^{2}=27 \times \frac{120}{9}=360

Answer key and solution verified before publishing.

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Exam
JEE Main 2026
Subject
Mathematics
Chapter
Vector Algebra
Topic
Collinearity and Coplanarity of Vectors and Points