CBSE Class 12 Math 2013 Solved Paper

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Question : 21
Total: 29
Find the coordinates of the point, where the line
x2
2
=
y+1
4
=
z2
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
r
.
^
i
+2
^
j
+3
^
k
4
= 0 and
r
.2
^
i
+
^
j
^
k
+5
= 0 and which is perpendicular to the plane
r
.5
^
i
+3
^
j
6
^
k
+8
= 0
Solution:  
The equation of the given line is
x2
2
=
y+1
4
=
z2
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 =
a
.
b
|
a
|
|
b
|
=
(3
^
i
+4
^
j
+2
^
k
)
.(
^
i
^
j
+
^
k
)
32+42+2212+12+12
=
34+2
293
=
1
87

⇒ θ = sin1(
1
87
)

Thus, the angle between the given line and the given plane is sin1(
1
87
)

OR
The equation of the given planes are
r
.
^
i
+2
^
j
+3
^
k
4
= 0 ... (1)
r
.2
^
i
+
^
j
^
k
+5
= 0 ... (2)
The equation of the plane passing through the intersection of the planes (1) and (2) is
|
r
.
^
i
+2
^
j
+3
^
k
4
|
+ λ |
r
.2
^
i
+
^
j
^
k
+5
|
= 4 - 5λ ... (3)
Given that plane (3) is perpendicular to the plane
r
.5
^
i
+3
^
j
6
^
k
+8
= 0
1 + 2λ × 5 + 2 + λ × 3 + 3 - λ × - 6 = 0
⇒ 19λ - 7 = 0
⇒ λ =
7
19

Putting λ =
7
19
in (3), we get
r
|(1+
14
19
)
^
i
+(2+
7
19
)
^
j
+(3
7
19
)
^
k
|
= 4 -
35
19

r
.(
33
19
^
i
+
45
19
^
j
+
50
19
^
k
)
=
41
19

r
.33
^
i
+45
^
j
+50
^
k
= 41. This is the equation of the required plane.
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