Join WhatsApp Icon JEE WhatsApp Group
Question 11

Let the position vectors of three vertices of a triangle be $$4\vec p+\vec q-3\vec r,\;-5\vec p+\vec q+2\vec r$$ and $$2\vec p-\vec q+2\vec r.$$ If the position vectors of the orthocenter and the circumcenter  of the triangle are $$\frac{\vec p+\vec q+\vec r}{4}$$ and $$\alpha\vec p+\beta\vec q+\gamma\vec r$$ { respectively, then $$\alpha+2\beta+5\gamma$$  is equal to:

Let the vertices of the triangle be $$P = 4\vec{p} + \vec{q} - 3\vec{r}$$, $$Q = -5\vec{p} + \vec{q} + 2\vec{r}$$, $$R = 2\vec{p} - \vec{q} + 2\vec{r}$$.

The orthocenter is $$H = \frac{\vec{p} + \vec{q} + \vec{r}}{4}$$.

We use the property that the centroid $$G$$ divides $$OH$$ in ratio $$1:2$$ (where $$O$$ is circumcenter), i.e., $$G$$ divides segment from circumcenter to orthocenter. More precisely: $$\vec{G} = \frac{\vec{P}+\vec{Q}+\vec{R}}{3}$$.

$$\vec{G} = \frac{(4-5+2)\vec{p} + (1+1-1)\vec{q} + (-3+2+2)\vec{r}}{3} = \frac{\vec{p} + \vec{q} + \vec{r}}{3}$$

Euler line relation: $$\vec{G} = \frac{\vec{H} + 2\vec{O}}{3}$$ where $$O$$ is the circumcenter.

$$\frac{\vec{p}+\vec{q}+\vec{r}}{3} = \frac{1}{3}\left(\frac{\vec{p}+\vec{q}+\vec{r}}{4} + 2\vec{O}\right)$$

$$\vec{p}+\vec{q}+\vec{r} = \frac{\vec{p}+\vec{q}+\vec{r}}{4} + 2\vec{O}$$

$$2\vec{O} = \frac{3(\vec{p}+\vec{q}+\vec{r})}{4}$$

$$\vec{O} = \frac{3}{8}(\vec{p}+\vec{q}+\vec{r}) = \frac{3}{8}\vec{p} + \frac{3}{8}\vec{q} + \frac{3}{8}\vec{r}$$

So $$\alpha = 3/8$$, $$\beta = 3/8$$, $$\gamma = 3/8$$.

$$\alpha + 2\beta + 5\gamma = \frac{3}{8} + \frac{6}{8} + \frac{15}{8} = \frac{24}{8} = 3$$.

The correct answer is Option A: 3.

Get AI Help

Create a FREE account and get:

  • Free JEE Mains Previous Papers PDF
  • Take JEE Mains paper tests

50,000+ JEE Students Trusted Our Score Calculator

Predict your JEE Main percentile, rank & performance in seconds

Ask AI

Ask our AI anything

AI can make mistakes. Please verify important information.