RD Sharma Class 12 Exercise 28.14 The Plane Solutions Maths - Download PDF Free Online

RD Sharma Class 12 Exercise 28.14 The Plane Solutions Maths - Download PDF Free Online

Updated on 25 Jan 2022, 11:53 AM IST

For a long time, RD Sharma books have set parameters for board exams. This book is essential in terms of exams because teachers often assign question papers from it. The Rd Sharma Class 12th Exercise 28.14 solutions that you will find here are a team of subject experts who constantly strive for student success. This particular exercise has three questions and few examples to clarify the concept in a better way. The chapter 'The Plane' discusses 2D surfaces having infinite dimensions with no thickness such that when two lines join them, both of them lie on the surface entirely and find the shortest distance between the lines.

The Plane Excercise: 28.14

The Plane Exercise 28.14 Question 1

Answer : $\frac{65}{\sqrt{122}}$
Hint : Distance between shortest lines
$D=\left|\frac{\left(\vec{a}_{2}-\vec{a}_{1}\right) \cdot\left(\vec{b}_{1} \times \vec{b}_{2}\right)}{\left|\vec{b}_{1} \times \vec{b_{2}}\right|}\right|$
Given : Find the shortest distance between lines $\frac{x-2}{-1}=\frac{y-5}{2}=\frac{z-0}{3}$ and $\frac{x-0}{2}=\frac{y+1}{-1}=\frac{z-1}{2}$
Solution : $\frac{x-2}{-1}=\frac{y-5}{2}=\frac{z-0}{3}$
$\vec{r}=(2 \hat{\imath}+5 \hat{\jmath})+\lambda(-\hat{\imath}+2 \hat{\jmath}+3 \hat{k}) \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \quad[\vec{r}=\vec{a}+\lambda \vec{b}]$
$\begin{aligned} &\vec{a}_{1}=2 \hat{i}+5 \hat{j} \\ &\vec{b}_{1}=-\hat{i}+2 \hat{j}+3 \hat{k} \end{aligned}$
$\frac{x-0}{2}=\frac{y+1}{-1}=\frac{z-1}{2}\; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \quad\left[\frac{x-x_{1}}{a}=\frac{y-y_{1}}{b}=\frac{z-z_{1}}{c}\right]$
$\begin{aligned} &\vec{r}=(-\hat{\jmath}+\hat{k})+\mu(2 \hat{\imath}-\hat{\jmath}+2 \hat{k}) \\ &\vec{a}_{2}=-\hat{j}+\hat{k}, \vec{b}_{2}=2 \hat{i}-\hat{j}+2 \hat{k} \\ &D=\left|\frac{\left.\overrightarrow{(a}_{2}-\vec{a}_{1}\right) \cdot\left(\vec{b}_{1} \times \vec{b}_{2}\right)}{\left|\vec{b}_{1} \times \vec{b}_{2}\right|}\right| \end{aligned}$
$\vec{b}_{1} \times \vec{b}_{2}=\left|\begin{array}{ccc} \hat{i} & \hat{j} & \hat{k} \\ -1 & 2 & 3 \\ 2 & -1 & 2 \end{array}\right|$
$\begin{aligned} &=\hat{i}(4+3)-\hat{j}(-2-6)+\hat{k}(1-4)=7 \hat{i}+8 \hat{j}-3 \hat{k} \\ &\left|\vec{b}_{1} \times \vec{b}_{2}\right|=\sqrt{7^{2}+8^{2}+(-3)^{2}}=\sqrt{49+64+9}=\sqrt{122} \\ &D=\left|\frac{(-2 \hat{\imath}-6 \hat{\jmath}+\hat{k}) \cdot(7 \hat{\imath}+8 \hat{\jmath}-3 \hat{k})}{\sqrt{122}}\right| \\ &=\left|\frac{-14-48-3}{\sqrt{122}}\right|=\frac{|-65|}{\sqrt{122}} \\ &D=\frac{65}{\sqrt{122}} \end{aligned}$

The Plane Exercise 28.14 Question 2

Answer : $2\sqrt{29}$
Hint: Formula of distance between shortest lines
$D=\left|\frac{\left(\vec{a}_{2}-\vec{a}_{1}\right) \cdot\left(\vec{b}_{1} \times \vec{b}_{2}\right)}{\left|\vec{b}_{1} \times \vec{b_{2}}\right|}\right|$
Given: Find the shortest distance between lines
$\frac{x+1}{7}=\frac{y+1}{6}=\frac{z+1}{1} \text { and } \frac{x-3}{1}=\frac{y-5}{-2}=\frac{z-7}{1}$
Solution : $\frac{x+1}{7}=\frac{y+1}{6}=\frac{z+1}{1}$
$\vec{r}=(-\hat{\imath}-\hat{\jmath}+\hat{k})+\lambda(7 \hat{\imath}-6 \hat{\jmath}+\hat{k}) \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \quad[\vec{r}=\vec{a}+\lambda \vec{b}]$
$\vec{a}=-\hat{i}-\hat{j}-\hat{k}, \vec{b}=7 \hat{i}-6 \hat{j}+\hat{k}$
Now $\frac{x-3}{1}=\frac{y-5}{-2}=\frac{z-7}{7} \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \quad\left[\frac{x-x_{1}}{a}=\frac{y-y_{1}}{b}=\frac{z-z_{1}}{c}\right]$
$\begin{aligned} &\vec{r}=(3 \hat{\imath}+5 \hat{\jmath}+7 \hat{k})+\lambda(\hat{\imath}-2 \hat{\jmath}+\hat{k}) \\ &\vec{a}_{2}=3 \hat{i}+5 \hat{j}+7 \hat{k}, \quad \vec{b}_{2}=\hat{i}-2 \hat{j}+\hat{k} \\ &\therefore\left(\vec{a}_{2}-\vec{a}_{1}\right)=(3 \hat{i}+5 \hat{j}+7 \hat{k})-(-\hat{i}-\hat{j}-\hat{k}) \\ &=4 \hat{i}+6 \hat{j}+8 \hat{k} \end{aligned}$
$\therefore \vec{b}_{1} \times \vec{b}_{2}=\left|\begin{array}{ccc} \hat{i} & \hat{j} & \hat{k} \\ 7 & -6 & 1 \\ 1 & -2 & 1 \end{array}\right|$
$=\hat{\imath}(-6+2)-\hat{\jmath}(7-1)+\hat{k}((-14)+6))$
$\begin{aligned} &\vec{b}_{1} \times \vec{b}_{2}=-4 \hat{i}-6 \hat{j}-8 \hat{k} \\ &D=\left|\frac{(4 \hat{i}+6 \hat{j}+8 \hat{k}) \cdot(-4 \hat{i}-6 \hat{j}-8 \hat{k})}{\sqrt{16+36+64}}\right| \\ &D=\left|\frac{-16-36-64}{\sqrt{122}}\right| \end{aligned}$
$\begin{aligned} &=\frac{-116}{\sqrt{116}}=\sqrt{116}=2 \sqrt{29} \\ &d=2 \sqrt{29} \text { (answer) } \end{aligned}$

The Plane Exercise 28.14 Question 3

Answer: $\frac{8}{\sqrt{14}}$
$\left|\frac{a x_{1}+b y_{1}+c z_{1}+d}{\sqrt{a^{2}+b^{2}+c^{2}}}\right|$
Given – Find the shortest distance between the lines $\frac{x-1}{2}=\frac{y-3}{4}=\frac{z+2}{1}$ and $3 x-y-2 z+4=0=2 x+y+z+1$
$\frac{x-1}{2}=\frac{y-3}{4}=\frac{z+2}{1}\; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \quad\left[\frac{x-x_{1}}{\alpha}=\frac{y-y_{1}}{b}=\frac{z-z_{1}}{c}\right]$
$x_{1}=1, y_{1}=3, z_{1}=-2$
length of perpendicular from $(1,3,-2)$ to the plane $3x-y-2z+4=0$
$\begin{aligned} &D=\left|\frac{a x_{1}+b y_{1}+c z_{1}+d}{\sqrt{a^{2}+b^{2}+c^{2}}}\right| \\ &D=\left|\frac{3(1)-1(3)-2(-2)+4}{\sqrt{(3)^{2}+(-1)^{2}+(-2)^{2}}}\right| \end{aligned}$
$\begin{aligned} &D=\left|\frac{8}{\sqrt{14}}\right| \\ &D=\frac{8}{\sqrt{14}} \text { (answer) } \end{aligned}$



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