Mathematics · Vector Algebra

JEE Main 2025 — 24 January, Evening Shift — Question 6

Let the position vectors of three vertices of a triangle be 4p→+q→−3r→,−5p→+q→+2r→4 \overrightarrow{\mathrm{p}}+\overrightarrow{\mathrm{q}}-3 \overrightarrow{\mathrm{r}},-5 \overrightarrow{\mathrm{p}}+\overrightarrow{\mathrm{q}}+2 \overrightarrow{\mathrm{r}} and 2p→−q→+2r→2 \overrightarrow{\mathrm{p}}-\overrightarrow{\mathrm{q}}+2 \overrightarrow{\mathrm{r}}.

If the position vectors of the orthocenter and the circumcenter of the triangle are p→+q→+r→4\frac{\overrightarrow{\mathrm{p}}+\overrightarrow{\mathrm{q}}+\overrightarrow{\mathrm{r}}}{4} and αp→+βq→+γr→\alpha \overrightarrow{\mathrm{p}}+\beta \overrightarrow{\mathrm{q}}+\gamma \overrightarrow{\mathrm{r}}

respectively, then α+2β+5γ\alpha+2 \beta+5 \gamma is equal to:

  1. Option A:

    3

    Correct
  2. Option B:

    1

  3. Option C:

    6

  4. Option D:

    4

Answer: A

Step-by-step solution

We know that O (orthocentre) p→+q→+r→4\frac{\overrightarrow{\mathrm{p}}+\overrightarrow{\mathrm{q}}+\overrightarrow{\mathrm{r}}}{4}

CC (circum centre) αp→+βq→+γr→\alpha \overrightarrow{\mathrm{p}}+\beta \overrightarrow{\mathrm{q}}+\gamma \overrightarrow{\mathrm{r}}

C(C( centroid )=p→+q→+r→3)=\frac{\overrightarrow{\mathrm{p}}+\overrightarrow{\mathrm{q}}+\overrightarrow{\mathrm{r}}}{3}

by relation

⇒2(αp→+βq→+γr→)+p→+q→+r→4=3(p→+q→+r→3)\Rightarrow 2(\alpha \overrightarrow{\mathrm{p}}+\beta \overrightarrow{\mathrm{q}}+\gamma \overrightarrow{\mathrm{r}})+\frac{\overrightarrow{\mathrm{p}}+\overrightarrow{\mathrm{q}}+\overrightarrow{\mathrm{r}}}{4}=3\left(\frac{\overrightarrow{\mathrm{p}}+\overrightarrow{\mathrm{q}}+\overrightarrow{\mathrm{r}}}{3}\right)

⇒8(αp⃗+βq→+γr→)=3(p→+q→+r→)\Rightarrow 8(\alpha \vec{p}+\beta \overrightarrow{\mathrm{q}}+\gamma \overrightarrow{\mathrm{r}})=3(\overrightarrow{\mathrm{p}}+\overrightarrow{\mathrm{q}}+\overrightarrow{\mathrm{r}})

⇒8α=3,8β=3,8γ=3\Rightarrow 8 \alpha=3,8 \beta=3,8 \gamma=3

α=38,β=38,γ=38\alpha=\frac{3}{8}, \beta=\frac{3}{8}, \gamma=\frac{3}{8}

∴α+2β+5γ\therefore \alpha+2 \beta+5 \gamma

38+68+158=248=3\frac{3}{8}+\frac{6}{8}+\frac{15}{8}=\frac{24}{8}=3

3\boxed3
Solution figure

Answer key and solution verified before publishing.

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Exam
JEE Main 2025
Subject
Mathematics
Chapter
Vector Algebra
Topic
Linear Combination of Vectors