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NDA 2015
Exam
NDA
Year
2015
Subject
Mathematics
Chapter
Three Dimensional Geometry

NDA 2015 · Mathematics · Three Dimensional Geometry

Let α, β, γ be distinct real numbers. The points with position...

Let α,β,γ\alpha, \beta, \gamma be distinct real numbers. The points with position vectors αi^+βj^+γk^,βi^+γj^+αk^\alpha \hat{i}+\beta \hat{j}+\gamma \hat{k}, \beta \hat{i}+\gamma \hat{j}+\alpha \hat{k} and γi^+αj^+βk^\gamma \hat{i}+\alpha \hat{j}+\beta \hat{k}

Options

  1. Aare collinear
  2. Bform an equilateral triangle
  3. Cform a scalene triangle
  4. Dform a right-angled triangle

Correct answer

B. form an equilateral triangle

Step-by-step solution

α,β\alpha, \beta and γ\gamma be distinct real numbers a⃗=αi^+βj^+γk^b⃗=βi^+γj^+αk^c⃗=γi^+αj^+βk^\begin{aligned} & \vec{a}=\alpha \hat{i}+\beta \hat{j}+\gamma \hat{k} \\ & \vec{b}=\beta \hat{i}+\gamma \hat{j}+\alpha \hat{k} \\ & \vec{c}=\gamma \hat{i}+\alpha \hat{j}+\beta \hat{k} \end{aligned} Let α=1,β=2\alpha=1, \beta=2 and α=3\alpha=3 then, a⃗=i^+2j^+3k^\quad \vec{a}=\hat{i}+2 \hat{j}+3 \hat{k}b→=2i^+3j^+k^c→=3i^+2j^+2k^\begin{aligned} & \overrightarrow{\mathrm{b}}=2 \hat{\mathrm{i}}+3 \hat{\mathrm{j}}+\hat{\mathrm{k}} \\ & \overrightarrow{\mathrm{c}}=3 \hat{\mathrm{i}}+2 \hat{\mathrm{j}}+2 \hat{\mathrm{k}} \end{aligned}$Now,Now,|\overrightarrow{\mathrm{ab}}|=\sqrt{(2-1)^2}+(3-2)^2+(1-3)^2=\sqrt{6}SimilarlySimilarly|\overrightarrow{\mathrm{bc}}|=|\overrightarrow{\mathrm{ac}}|=\sqrt{6}$ Hence, point for equilateral triangle.
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