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Need solution for RD Sharma Maths Class 12 Chapter 28 The Plane Excercise 28.11 Question 26

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Answer: Required equation of plane is  x-y+z=0  and angle  \sin \theta=\frac{-1}{\sqrt{3}}

Hint: Use formula  \sin \theta=\frac{\vec{n} \cdot \vec{b}}{|\vec{n}||\vec{b}|}

Given: Point  (2,1,-1) \text { and } 2 x+y-z=3 \text { and } x+2 y+z=2

Solution:

Line of intersection  \mathrm{n}_{1} \times \mathrm{n}_{2}

                \vec{A} \vec{R} \cdot\left[n_{1} \times n_{2}\right]=0

Scalar triple product, \left[\begin{array}{lll} \overrightarrow{A R} & n_{1} & n_{2} \end{array}\right]

\overrightarrow{A R}= Position Vector of \overrightarrow{ R} – Position Vector of  \overrightarrow{ A}

        \begin{aligned} &=(x-2) \hat{i}+(y-1) \hat{j}+(z+1) \hat{k} \\ & \end{aligned}

        \left|\begin{array}{ccc} x-2 & y-1 & z+1 \\ 2 & 1 & -1 \\ 1 & 2 & 1 \end{array}\right|=0

        \begin{aligned} &(x-2)(1+2)-(y-1)(2+1)+(z+1)(4-1)=0 \\ & \end{aligned}

        x-2-y+1+z+1=0

        x-y+z=0  is required equation of plane

Now, the angle between the plane and the y-axis is  \sin \theta=\frac{\vec{n} \cdot \vec{b}}{|\vec{n}||\vec{b}|}

\begin{aligned} &\sin \theta=\frac{-1}{\sqrt{1+1+1} \sqrt{0+1+0}} \\ & \end{aligned}

\sin \theta=\frac{-1}{\sqrt{3}} \\                                      \left[\sin \theta=\frac{\vec{n} \cdot \vec{b}}{|\vec{n}||\vec{b}|}\right]

\theta=\sin ^{-1} \frac{-1}{\sqrt{3}}

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