JEE Advance - Mathematics (2023 - Paper 1 Online - No. 12)
Let $P$ be the plane $\sqrt{3} x+2 y+3 z=16$ and let
$S=\left\{\alpha \hat{i}+\beta \hat{j}+\gamma \hat{k}: \alpha^2+\beta^2+\gamma^2=1\right.$ and the distance of $(\alpha, \beta, \gamma)$ from the plane $P$ is $\left.\frac{7}{2}\right\}$.
Let $\vec{u}, \vec{v}$ and $\vec{w}$ be three distinct vectors in $S$ such that $|\vec{u}-\vec{v}|=|\vec{v}-\vec{w}|=|\vec{w}-\vec{u}|$. Let $V$ be the volume of the parallelepiped determined by vectors $\vec{u}, \vec{v}$ and $\vec{w}$. Then the value of $\frac{80}{\sqrt{3}} V$ is :
Answer
45
Explanation
$\begin{aligned} & P: \sqrt{3} x+2 y+3 z=16 \\\\ & \qquad S=\left\{\alpha \hat{\imath}+\beta \hat{j}+\gamma \hat{k}: \alpha^2+\beta^2+\gamma^2=1,\right. \left.\quad d_p=\frac{7}{2}\right\} \\\\ & \because|\vec{u}-\vec{v}|=|\vec{v}-\vec{w}|=|\vec{w}-\vec{u}| .............. (1)\end{aligned}$
$\vec{u}, \vec{v}, \vec{w}$ are elements of set $S$ and in set $S$ magnitude of vector is 1
$\therefore \vec{u}, \vec{v}, \vec{w}$ are unit vectors and by equation (1) we can system $\vec{u}, \vec{v}, \vec{w}$ are equally inclined and vertices of equilateral triangle also lying on a circle which is intersection of sphere $|\vec{r}|=1$
Distance from origin to $P$,
$$ d=\frac{|-16|}{\sqrt{3+4+9}}=\frac{16}{4}=4 $$
$\therefore$ Plane containing $\hat{u}, \hat{v}, \hat{w}$ are at a distance
$4-\frac{7}{2}=\frac{1}{2}$ from origin and Parallel to $\sqrt{3 x}+2 y+3 z$ $=16$.
$\therefore$ Equation of the plane is
$$ \begin{array}{rlrl} & \sqrt{3 x}+2 y+3 z & =\gamma \\\\ & \therefore \frac{1}{2} =\left|\frac{\gamma}{4}\right| \\\\ & \Rightarrow \gamma = \pm 2 \\\\ & \sqrt{3 x}+2 y+3 z =2 \end{array} $$
Equation of sphere $x^2+y^2+z^2=1$
$\therefore$ Radius or circle
$\begin{aligned} r & =\sqrt{1-\frac{1}{4}}=\frac{\sqrt{3}}{2} \\\\ \text { then } \quad \frac{a}{2} & =\frac{\sqrt{3}}{2} \cos 30^{\circ} \\\\ a & =\sqrt{3} \times \frac{\sqrt{3}}{2}=\frac{3}{2}\end{aligned}$
$\therefore$ Area or triangle
$$ =\frac{\sqrt{3}}{2} a^2=\frac{\sqrt{3}}{2} \times \frac{9}{4}=\frac{9 \sqrt{3}}{16} $$
$\therefore$ Volume of Parallelepiped
$$ \begin{aligned} & =2 \times \frac{1}{2} \times \frac{9 \sqrt{3}}{16} \\\\ V & =\frac{9 \sqrt{3}}{16} \end{aligned} $$
$\therefore$ Volume of Parallelepiped
$$ \begin{aligned} = & 2 \times \frac{1}{2} \times \frac{9 \sqrt{3}}{16} \\\\ V & =\frac{9 \sqrt{3}}{16} \end{aligned} $$
$\therefore \frac{80 \mathrm{~V}}{\sqrt{3}}=\frac{80}{\sqrt{3}} \times \frac{9 \sqrt{3}}{16}=45$
$\vec{u}, \vec{v}, \vec{w}$ are elements of set $S$ and in set $S$ magnitude of vector is 1
$\therefore \vec{u}, \vec{v}, \vec{w}$ are unit vectors and by equation (1) we can system $\vec{u}, \vec{v}, \vec{w}$ are equally inclined and vertices of equilateral triangle also lying on a circle which is intersection of sphere $|\vec{r}|=1$
Distance from origin to $P$,
$$ d=\frac{|-16|}{\sqrt{3+4+9}}=\frac{16}{4}=4 $$
$\therefore$ Plane containing $\hat{u}, \hat{v}, \hat{w}$ are at a distance
$4-\frac{7}{2}=\frac{1}{2}$ from origin and Parallel to $\sqrt{3 x}+2 y+3 z$ $=16$.
$\therefore$ Equation of the plane is
$$ \begin{array}{rlrl} & \sqrt{3 x}+2 y+3 z & =\gamma \\\\ & \therefore \frac{1}{2} =\left|\frac{\gamma}{4}\right| \\\\ & \Rightarrow \gamma = \pm 2 \\\\ & \sqrt{3 x}+2 y+3 z =2 \end{array} $$
Equation of sphere $x^2+y^2+z^2=1$
$\therefore$ Radius or circle

$\begin{aligned} r & =\sqrt{1-\frac{1}{4}}=\frac{\sqrt{3}}{2} \\\\ \text { then } \quad \frac{a}{2} & =\frac{\sqrt{3}}{2} \cos 30^{\circ} \\\\ a & =\sqrt{3} \times \frac{\sqrt{3}}{2}=\frac{3}{2}\end{aligned}$

$\therefore$ Area or triangle
$$ =\frac{\sqrt{3}}{2} a^2=\frac{\sqrt{3}}{2} \times \frac{9}{4}=\frac{9 \sqrt{3}}{16} $$
$\therefore$ Volume of Parallelepiped
$$ \begin{aligned} & =2 \times \frac{1}{2} \times \frac{9 \sqrt{3}}{16} \\\\ V & =\frac{9 \sqrt{3}}{16} \end{aligned} $$
$\therefore$ Volume of Parallelepiped
$$ \begin{aligned} = & 2 \times \frac{1}{2} \times \frac{9 \sqrt{3}}{16} \\\\ V & =\frac{9 \sqrt{3}}{16} \end{aligned} $$
$\therefore \frac{80 \mathrm{~V}}{\sqrt{3}}=\frac{80}{\sqrt{3}} \times \frac{9 \sqrt{3}}{16}=45$
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