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Need solution for RD Sharma Maths Class 12 Chapter 28 The Plane Excercise 28.11 Question 21

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Answer:  \overrightarrow{\mathrm{r}}=(\hat{\mathrm{i}}+2 \hat{\mathrm{j}}+3 \hat{\mathrm{k}})+\mathrm{m}(\hat{\mathrm{i}}+2 \hat{\mathrm{j}}-5 \hat{\mathrm{k}})

Hint:  \overrightarrow{\mathrm{r}}=\overrightarrow{\mathrm{a}}+\mathrm{kb}

Given:  \vec{r} \cdot(\hat{i}+2 \hat{j}-5 \hat{k})+9=0 \text { and }(1,2,3)

Solution: We know that equation of line passing through point  \vec{a} and parallel to vector \vec{b}  is given by  \vec{r}=\vec{a}+k \vec{b}                                 …………. (1)

Given that, the line is passing through  \left ( 1,2,3 \right )

            So,  \vec{a}=\hat{i}+2 \hat{j}+3 \hat{k}

It is given that line is perpendicular to plane  \vec{r} \cdot(\hat{i}+2 \hat{j}-5 \hat{k})+9=0

So, normal to plane \left ( \vec{n} \right ) is parallel to  \vec{b}

So, Let  \vec{b}=\vec{n}=1(\hat{i}+2 \hat{j}-5 \hat{k})

Putting \vec{a} \& \vec{b}  in (1), equation of line is

            \begin{aligned} &\vec{r}=(\hat{i}+2 \hat{j}+3 \hat{k})+k\{1(\hat{i}+2 \hat{j}-5 \hat{k})\} \\ & \end{aligned}

            \Rightarrow \vec{r}=(\hat{i}+2 \hat{j}+3 \hat{k})+m(\hat{i}+2 \hat{j}-5 \hat{k})

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