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CBSE Class 12 Math 2013 Solved Paper

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Question : 21 of 29
Marks: +1, -0
Find the coordinates of the point, where the line x−22\frac{x-2}{2} = y+14\frac{y+1}{4} = z−22\frac{z-2}{2} intersects the plane x – y + z – 5 = 0. Also find the angle between the line and the plane.
OR
Find the vector equation of the plane which contains the line of intersection of the planes r⃗⋅i^+2j^+3k^−4\vec{r} \cdot \hat{i} + \widehat{2j} + \widehat{3k} - 4 = 0 and r⃗⋅2i^+j^−k^+5\vec{r} \cdot \widehat{2i} + \hat{j} - \hat{k} + 5 = 0 and which is perpendicular to the plane r⃗⋅5i^+3j^−6k^+8\vec{r} \cdot \widehat{5i} + \widehat{3j} - \widehat{6k} + 8 = 0
Solution:  
The equation of the given line is x−22\frac{x-2}{2} = y+14\frac{y+1}{4} = z−22\frac{z-2}{2} ... (1)
Any point on the given line is (3λ + 2, 4λ - 1 , 2λ + 2)
If this point lies on the given plane x – y + z – 5 = 0, then
3λ + 2 -(4λ - 1) + 2λ + 2 - 5 = 0
⇒ λ = 0
Putting λ = 0 in (3λ + 2, 4λ - 1 , 2λ + 2) , we get the point of intersection of the given line and the plane is (2, -1, 2).
Let θ be the angle between the given line and the plane
∴ sin θ = a⃗⋅b⃗∣a⃗∣∣b⃗∣\frac{\vec{a} \cdot \vec{b}}{\left|\vec{a}\right|\left|\vec{b}\right|} = (3i^+4j^+2k^)⋅(i^−j^+k^)32+42+2212+12+12\frac{(\widehat{3i} + \widehat{4j} + \widehat{2k}) \cdot (\hat{i} - \hat{j} + \hat{k})}{\sqrt{3^2+4^2+2^2} \sqrt{1^2+1^2+1^2}} = 3−4+2293\frac{3 - 4 + 2}{\sqrt{29} \sqrt{3}} = 187\frac{1}{\sqrt{87}}
⇒ θ = sin⁡−1(187)\sin^{-1}\left(\frac{1}{\sqrt{87}}\right)
Thus, the angle between the given line and the given plane is sin⁡−1(187)\sin^{-1}\left(\frac{1}{\sqrt{87}}\right)
OR
The equation of the given planes are
r⃗⋅i^+2j^+3k^−4\vec{r} \cdot \hat{i} + \widehat{2j} + \widehat{3k} - 4 = 0 ... (1)
r⃗⋅2i^+j^−k^+5\vec{r} \cdot \widehat{2i} + \hat{j} - \hat{k} + 5 = 0 ... (2)
The equation of the plane passing through the intersection of the planes (1) and (2) is
∣r⃗⋅i^+2j^+3k^−4∣\left|\vec{r} \cdot \hat{i} + \widehat{2j} + \widehat{3k} - 4\right| + λ ∣r⃗⋅2i^+j^−k^+5∣\left|\vec{r} \cdot \widehat{2i} + \hat{j} - \hat{k} + 5\right| = 4 - 5λ ... (3)
Given that plane (3) is perpendicular to the plane r⃗⋅5i^+3j^−6k^+8\vec{r} \cdot \widehat{5i} + \widehat{3j} - \widehat{6k} + 8 = 0
1 + 2λ × 5 + 2 + λ × 3 + 3 - λ × - 6 = 0
⇒ 19λ - 7 = 0
⇒ λ = 719\frac{7}{19}
Putting λ = 719\frac{7}{19} in (3), we get
r⃗∣(1+1419)i^+(2+719)j^+(3−719)k^∣\vec{r} \left|\left(1+\frac{14}{19}\right)\hat{i} + \left(2+\frac{7}{19}\right)\hat{j} + \left(3-\frac{7}{19}\right)\hat{k}\right|
= 4 - 3519\frac{35}{19}
⇒ r⃗⋅(3319i^+4519j^+5019k^)\vec{r} \cdot \left(\frac{33}{19}\hat{i} + \frac{45}{19}\hat{j} + \frac{50}{19}\hat{k}\right) = 4119\frac{41}{19}
⇒ r⃗⋅33i^+45j^+50k^\vec{r} \cdot \widehat{33i} + \widehat{45j} + \widehat{50k} = 41. This is the equation of the required plane.
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