JEE Advanced · Mathematics · 31. 3D Geometry
Let \(L_1\) be the line of intersection of the planes given by the equation- \(2 x+3 y+z=4 \text { and } x+2 y+z=5 .\)
Let \(L_2\) be the line passing through the point \(P(2,-1,3)\) and parallel to \(L_1\). Let \(M\) denote the plane given by the equation- \(2 x+y-2 z=6\)
Suppose that the line \(L_2\) meets the plane \(M\) at the point \(Q\). Let \(R\) be the foot of the perpendicular drawn from \(P\) to the plane \(M\).
Then which of the following statements is (are) TRUE?
- A The length of the line segment \(P Q\) is \(9 \sqrt{3}\)
- B The length of the line segment \(Q R\) is 15
- C The area of \(\triangle P Q R\) is \(\frac{3}{2} \sqrt{234}\)
- D The acute angle between the line segments \(P Q\) and \(P R\) is \(\cos ^{-1}\left(\frac{1}{2 \sqrt{3}}\right)\)
Answer & Solution
Correct Answer
(C) The area of \(\triangle P Q R\) is \(\frac{3}{2} \sqrt{234}\)
Step-by-step Solution
Detailed explanation
Let \(L_1: \vec{r}=\vec{a}+t \vec{b}\)
\(\overrightarrow{\mathrm{b}}=\left|\begin{array}{ccc}\hat{\mathrm{i}} & \hat{\mathrm{j}} & \hat{\mathrm{k}} \\ 2 & 3 & 1 \\ 1 & 2 & 1\end{array}\right|=\hat{\mathrm{i}}(1)-\hat{\mathrm{j}}(1)+\hat{\mathrm{k}}(1)\)
Dr's of \(L_1: <1,-1,1>\)
\(\begin{aligned}
& L_1: \frac{x+1}{1}=\frac{y-0}{-1}=\frac{z-6}{1} \\
& \& L_2: \frac{x-2}{1}=\frac{y+1}{-1}=\frac{z-3}{1}=\lambda \\
& M: 2 x+y-2 z-6=0
\end{aligned}\)
Let point on \(\mathrm{L}_2(\lambda+2,-\lambda-1, \lambda+3)\)
\(\begin{aligned}
& 2(\lambda+2)-\lambda-1-2 \lambda-6-6=0 \\
& 2 \lambda+4-3 \lambda-13=0 \\
& \lambda=-9 \\
& \therefore Q(-7,8,-6)
\end{aligned}\)

Line PR: \(\frac{x-2}{2}=\frac{y+1}{1}=\frac{z-3}{-2}=\mu\)
\(\mathrm{R}(2 \mu+2, \mu-1,-2 \mu+3)\)
Put in plane
\(\begin{aligned}
& 2(2 \mu+2)+\mu-1-2(-2 \mu+3)-6=0 \\
& 4 \mu+4+\mu-1+4 \mu-6-6=0 \\
& 9 \mu-9=0 \Rightarrow \mu=1 \\
& \mathrm{R}(4,0,1) \\
& \mathrm{P}(2,-1,3) \& \mathrm{Q}(-7,8,-6) \\
& \mathrm{PQ}=\sqrt{81+81+81}=9 \sqrt{3} \\
& \mathrm{Q}(-7,8,-6) \& \mathrm{R}(4,0,1)
\end{aligned}\)
\(\mathrm{QR}=\sqrt{121+64+49}=\sqrt{234}\)
Area ( \(\triangle \mathrm{PQR}\) )
\(\begin{aligned}
& =\frac{1}{2}|\overrightarrow{\mathrm{QP}} \times \overrightarrow{\mathrm{QR}}| \\
& =\frac{1}{2}\left|\begin{array}{ccc}
\hat{\mathrm{i}} & \hat{\mathrm{j}} & \hat{\mathrm{k}} \\
9 & -9 & 9 \\
11 & -8 & 7
\end{array}\right| \\
& =\frac{3}{2} \sqrt{234} \\
& \overrightarrow{\mathrm{PQ}}=-9 \hat{\mathrm{i}}+9 \hat{\mathrm{j}}-9 \hat{\mathrm{k}}=-9(\hat{\mathrm{i}}-\hat{\mathrm{j}}+\hat{\mathrm{k}}) \\
& \overrightarrow{\mathrm{PR}}=2 \hat{\mathrm{i}}+\hat{\mathrm{j}}-2 \hat{\mathrm{k}}
\end{aligned}\)
\(\begin{aligned} & \cos \theta=\left|\frac{\overrightarrow{\mathrm{PQ}} \cdot \overrightarrow{\mathrm{PR}}}{\mathrm{PQ} \cdot \mathrm{PR}}\right| \\ & =\frac{9}{9 \sqrt{3} \times 3}=\frac{1}{3 \sqrt{3}} \\ & \theta=\cos ^{-1}\left(\frac{1}{3 \sqrt{3}}\right)\end{aligned}\)
\(\overrightarrow{\mathrm{b}}=\left|\begin{array}{ccc}\hat{\mathrm{i}} & \hat{\mathrm{j}} & \hat{\mathrm{k}} \\ 2 & 3 & 1 \\ 1 & 2 & 1\end{array}\right|=\hat{\mathrm{i}}(1)-\hat{\mathrm{j}}(1)+\hat{\mathrm{k}}(1)\)
Dr's of \(L_1: <1,-1,1>\)
\(\begin{aligned}
& L_1: \frac{x+1}{1}=\frac{y-0}{-1}=\frac{z-6}{1} \\
& \& L_2: \frac{x-2}{1}=\frac{y+1}{-1}=\frac{z-3}{1}=\lambda \\
& M: 2 x+y-2 z-6=0
\end{aligned}\)
Let point on \(\mathrm{L}_2(\lambda+2,-\lambda-1, \lambda+3)\)
\(\begin{aligned}
& 2(\lambda+2)-\lambda-1-2 \lambda-6-6=0 \\
& 2 \lambda+4-3 \lambda-13=0 \\
& \lambda=-9 \\
& \therefore Q(-7,8,-6)
\end{aligned}\)

Line PR: \(\frac{x-2}{2}=\frac{y+1}{1}=\frac{z-3}{-2}=\mu\)
\(\mathrm{R}(2 \mu+2, \mu-1,-2 \mu+3)\)
Put in plane
\(\begin{aligned}
& 2(2 \mu+2)+\mu-1-2(-2 \mu+3)-6=0 \\
& 4 \mu+4+\mu-1+4 \mu-6-6=0 \\
& 9 \mu-9=0 \Rightarrow \mu=1 \\
& \mathrm{R}(4,0,1) \\
& \mathrm{P}(2,-1,3) \& \mathrm{Q}(-7,8,-6) \\
& \mathrm{PQ}=\sqrt{81+81+81}=9 \sqrt{3} \\
& \mathrm{Q}(-7,8,-6) \& \mathrm{R}(4,0,1)
\end{aligned}\)
\(\mathrm{QR}=\sqrt{121+64+49}=\sqrt{234}\)
Area ( \(\triangle \mathrm{PQR}\) )
\(\begin{aligned}
& =\frac{1}{2}|\overrightarrow{\mathrm{QP}} \times \overrightarrow{\mathrm{QR}}| \\
& =\frac{1}{2}\left|\begin{array}{ccc}
\hat{\mathrm{i}} & \hat{\mathrm{j}} & \hat{\mathrm{k}} \\
9 & -9 & 9 \\
11 & -8 & 7
\end{array}\right| \\
& =\frac{3}{2} \sqrt{234} \\
& \overrightarrow{\mathrm{PQ}}=-9 \hat{\mathrm{i}}+9 \hat{\mathrm{j}}-9 \hat{\mathrm{k}}=-9(\hat{\mathrm{i}}-\hat{\mathrm{j}}+\hat{\mathrm{k}}) \\
& \overrightarrow{\mathrm{PR}}=2 \hat{\mathrm{i}}+\hat{\mathrm{j}}-2 \hat{\mathrm{k}}
\end{aligned}\)
\(\begin{aligned} & \cos \theta=\left|\frac{\overrightarrow{\mathrm{PQ}} \cdot \overrightarrow{\mathrm{PR}}}{\mathrm{PQ} \cdot \mathrm{PR}}\right| \\ & =\frac{9}{9 \sqrt{3} \times 3}=\frac{1}{3 \sqrt{3}} \\ & \theta=\cos ^{-1}\left(\frac{1}{3 \sqrt{3}}\right)\end{aligned}\)
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