Page 1
Edurev123
3. Plane and its Properties
3.1 Find the equations of the straight line through the point (?? ,?? ,?? ) to intersect the
straight line
?? +?? =?? +?? =?? (?? -?? )
and parallel to the plane ?? ?? +?? +?? ?? =?? .
(2011 : 10 Marks)
Solution:
Let the required line intersects the given line
?? +?? =?? +1=2(?? -2) (??)
at (?? 1
,?? 1
,?? 1
)
? The equations of the required line passing through (2,1,2) and (?? 1
,?? 1
,?? 1
) are
?? -3
?? 1
-3
=
?? -1
?? 1
-1
=
?? -2
?? 1
-1
(???? )
The direction ratios of the line (ii) are ?? 1
-3,?? 1
-1,?? 1
-1.
Because the line (ii) is parallel to the plane 4?? +?? +5?? =0, therefore the normal to the
plane with direction ratios 4,1,5 is perpendicular to the line (ii).
? 4(?? 1
-3)+1(?? 1
-1)+5(?? 1
-1)=0
or 4?? 1
+?? 2
+5?? 2
-23=0 (?????? )
Because the point (?? 1
,?? 1
,?? 1
) lies on (i),
? ?? 1
+4=?? 1
+1=2(?? 1
-2)
? ?? 1
=2?? 1
-8,?? 1
=2?? 1
-5 (iv)
? from (iv),we get
4(2?? 1
-8)+2?? 1
-5+5?? 1
-23 =0
? ?? 1
=4
Page 2
Edurev123
3. Plane and its Properties
3.1 Find the equations of the straight line through the point (?? ,?? ,?? ) to intersect the
straight line
?? +?? =?? +?? =?? (?? -?? )
and parallel to the plane ?? ?? +?? +?? ?? =?? .
(2011 : 10 Marks)
Solution:
Let the required line intersects the given line
?? +?? =?? +1=2(?? -2) (??)
at (?? 1
,?? 1
,?? 1
)
? The equations of the required line passing through (2,1,2) and (?? 1
,?? 1
,?? 1
) are
?? -3
?? 1
-3
=
?? -1
?? 1
-1
=
?? -2
?? 1
-1
(???? )
The direction ratios of the line (ii) are ?? 1
-3,?? 1
-1,?? 1
-1.
Because the line (ii) is parallel to the plane 4?? +?? +5?? =0, therefore the normal to the
plane with direction ratios 4,1,5 is perpendicular to the line (ii).
? 4(?? 1
-3)+1(?? 1
-1)+5(?? 1
-1)=0
or 4?? 1
+?? 2
+5?? 2
-23=0 (?????? )
Because the point (?? 1
,?? 1
,?? 1
) lies on (i),
? ?? 1
+4=?? 1
+1=2(?? 1
-2)
? ?? 1
=2?? 1
-8,?? 1
=2?? 1
-5 (iv)
? from (iv),we get
4(2?? 1
-8)+2?? 1
-5+5?? 1
-23 =0
? ?? 1
=4
? from (iv), ?? 1
=0,?? 1
=3
? from (ii), the equations of the required line are :
?? -3
-3
=
?? -1
2
=
?? 2
-2
2
3.2 Find the equation of the plane which passes through the points (?? ,?? ,?? ) and
(?? ,?? ,-?? ) and is parallel to the line joining the points (-?? ,?? ,-?? ),(?? ,-?? ,?? ) . Find also
the distance between the line and the plane.
(2013 : 10 Marks)
Solution:
The general equation of plane through (0,1,1)
??(?? -0)+?? (?? -1)+?? (?? -1)=0
???? +?? (?? -1)+?? (?? -1)=0
(??)
(2,0,-1) lies on this plane.
? 2?? -?? -2?? =0
Direction ratios of line passing through (-1,1,-2),(3,-2,4) .
?? 1
=3-1=4,?? ?? =(-2-1)=-3;?? 1
=(4-(-2))=6
? Direction ratios are 4::-3::6
This is parallel to plane.
? 4?? -3?? +6?? =0
From (ii) and (iii)
?? -12
=
?? -20
=
?? -2
??? :?? :?? ::6:10:1
? Equation of plane is
6?? +10(?? -1)+(?? -1) =0
? 6?? +10?? +?? -11 =0
Equation of plane parallel to this plane and passing through line
6(?? +1)+10(?? -1)+1(?? +2)=0
6?? +10?? +?? -2=0
Page 3
Edurev123
3. Plane and its Properties
3.1 Find the equations of the straight line through the point (?? ,?? ,?? ) to intersect the
straight line
?? +?? =?? +?? =?? (?? -?? )
and parallel to the plane ?? ?? +?? +?? ?? =?? .
(2011 : 10 Marks)
Solution:
Let the required line intersects the given line
?? +?? =?? +1=2(?? -2) (??)
at (?? 1
,?? 1
,?? 1
)
? The equations of the required line passing through (2,1,2) and (?? 1
,?? 1
,?? 1
) are
?? -3
?? 1
-3
=
?? -1
?? 1
-1
=
?? -2
?? 1
-1
(???? )
The direction ratios of the line (ii) are ?? 1
-3,?? 1
-1,?? 1
-1.
Because the line (ii) is parallel to the plane 4?? +?? +5?? =0, therefore the normal to the
plane with direction ratios 4,1,5 is perpendicular to the line (ii).
? 4(?? 1
-3)+1(?? 1
-1)+5(?? 1
-1)=0
or 4?? 1
+?? 2
+5?? 2
-23=0 (?????? )
Because the point (?? 1
,?? 1
,?? 1
) lies on (i),
? ?? 1
+4=?? 1
+1=2(?? 1
-2)
? ?? 1
=2?? 1
-8,?? 1
=2?? 1
-5 (iv)
? from (iv),we get
4(2?? 1
-8)+2?? 1
-5+5?? 1
-23 =0
? ?? 1
=4
? from (iv), ?? 1
=0,?? 1
=3
? from (ii), the equations of the required line are :
?? -3
-3
=
?? -1
2
=
?? 2
-2
2
3.2 Find the equation of the plane which passes through the points (?? ,?? ,?? ) and
(?? ,?? ,-?? ) and is parallel to the line joining the points (-?? ,?? ,-?? ),(?? ,-?? ,?? ) . Find also
the distance between the line and the plane.
(2013 : 10 Marks)
Solution:
The general equation of plane through (0,1,1)
??(?? -0)+?? (?? -1)+?? (?? -1)=0
???? +?? (?? -1)+?? (?? -1)=0
(??)
(2,0,-1) lies on this plane.
? 2?? -?? -2?? =0
Direction ratios of line passing through (-1,1,-2),(3,-2,4) .
?? 1
=3-1=4,?? ?? =(-2-1)=-3;?? 1
=(4-(-2))=6
? Direction ratios are 4::-3::6
This is parallel to plane.
? 4?? -3?? +6?? =0
From (ii) and (iii)
?? -12
=
?? -20
=
?? -2
??? :?? :?? ::6:10:1
? Equation of plane is
6?? +10(?? -1)+(?? -1) =0
? 6?? +10?? +?? -11 =0
Equation of plane parallel to this plane and passing through line
6(?? +1)+10(?? -1)+1(?? +2)=0
6?? +10?? +?? -2=0
? distance between line and plane
= distance between two plane
=
|?? 1
-?? 2
|
v
??˜2
+?? 2
+?? 2
=
9
v6
2
+10
2
+?? 2
This must be on the plane for the intersection of this sphere to be a great circle.
? 5(
5?? -3
2
)-(4-2?? )+2(4?? -2)+7=0
? 45?? =17??? =
17
45
Page 4
Edurev123
3. Plane and its Properties
3.1 Find the equations of the straight line through the point (?? ,?? ,?? ) to intersect the
straight line
?? +?? =?? +?? =?? (?? -?? )
and parallel to the plane ?? ?? +?? +?? ?? =?? .
(2011 : 10 Marks)
Solution:
Let the required line intersects the given line
?? +?? =?? +1=2(?? -2) (??)
at (?? 1
,?? 1
,?? 1
)
? The equations of the required line passing through (2,1,2) and (?? 1
,?? 1
,?? 1
) are
?? -3
?? 1
-3
=
?? -1
?? 1
-1
=
?? -2
?? 1
-1
(???? )
The direction ratios of the line (ii) are ?? 1
-3,?? 1
-1,?? 1
-1.
Because the line (ii) is parallel to the plane 4?? +?? +5?? =0, therefore the normal to the
plane with direction ratios 4,1,5 is perpendicular to the line (ii).
? 4(?? 1
-3)+1(?? 1
-1)+5(?? 1
-1)=0
or 4?? 1
+?? 2
+5?? 2
-23=0 (?????? )
Because the point (?? 1
,?? 1
,?? 1
) lies on (i),
? ?? 1
+4=?? 1
+1=2(?? 1
-2)
? ?? 1
=2?? 1
-8,?? 1
=2?? 1
-5 (iv)
? from (iv),we get
4(2?? 1
-8)+2?? 1
-5+5?? 1
-23 =0
? ?? 1
=4
? from (iv), ?? 1
=0,?? 1
=3
? from (ii), the equations of the required line are :
?? -3
-3
=
?? -1
2
=
?? 2
-2
2
3.2 Find the equation of the plane which passes through the points (?? ,?? ,?? ) and
(?? ,?? ,-?? ) and is parallel to the line joining the points (-?? ,?? ,-?? ),(?? ,-?? ,?? ) . Find also
the distance between the line and the plane.
(2013 : 10 Marks)
Solution:
The general equation of plane through (0,1,1)
??(?? -0)+?? (?? -1)+?? (?? -1)=0
???? +?? (?? -1)+?? (?? -1)=0
(??)
(2,0,-1) lies on this plane.
? 2?? -?? -2?? =0
Direction ratios of line passing through (-1,1,-2),(3,-2,4) .
?? 1
=3-1=4,?? ?? =(-2-1)=-3;?? 1
=(4-(-2))=6
? Direction ratios are 4::-3::6
This is parallel to plane.
? 4?? -3?? +6?? =0
From (ii) and (iii)
?? -12
=
?? -20
=
?? -2
??? :?? :?? ::6:10:1
? Equation of plane is
6?? +10(?? -1)+(?? -1) =0
? 6?? +10?? +?? -11 =0
Equation of plane parallel to this plane and passing through line
6(?? +1)+10(?? -1)+1(?? +2)=0
6?? +10?? +?? -2=0
? distance between line and plane
= distance between two plane
=
|?? 1
-?? 2
|
v
??˜2
+?? 2
+?? 2
=
9
v6
2
+10
2
+?? 2
This must be on the plane for the intersection of this sphere to be a great circle.
? 5(
5?? -3
2
)-(4-2?? )+2(4?? -2)+7=0
? 45?? =17??? =
17
45
? The given sphere is
?? 2
+?? 2
+?? 2
-
10
9
?? +
146
45
?? -
22
45
?? -
106
45
=0
? 45(?? 2
+?? 2
+?? 2
)-50?? +146?? -22?? -106=0
3.3 Obtain the equation of the plane passing through the points (?? ,?? ,?? ) and
(?? ,-?? ,?? ) parallei to ?? -axis.
(2015: 6 Marks)
Solution:
The equation of any plane through (2,3,1) is
?? (?? -2)+?? (?? -3)+?? (?? -1)=0 (??)
It passes through (4,-5,3)
? ?? (4-2)+?? (-5-3)+?? (3-1)=0
i.e., ?? -4?? +?? =0 (???? )
If the plane (i) is parallel to ?? -axis, then it is perpendicular to ???? -plar 3, i.e., ?? =0, i.e.,
? 1?? +0?? +0?? =0
? from (ii), 1?? +0?? +0?? =0??? =0
?
?? 0
=
?? 1
=
?? 4
Hence, (i) becomes 0+1(?? -3)+4(?? -1)=0
?? +4?? -7=0
3.4 Find the surface generated by a line which intersects the lines ?? =?? =?? ,?? +
?? ?? =?? =?? +?? and parallel to the plane ?? +?? =?? .
(2016 : 10 Marks)
Solution:
Topic : Equation of a straight line intersecting two given lines.
Given lines are :
?? -?? =0=?? -?? (??)
?? +3?? -?? =0=?? +?? -?? (???? )
Hence, the equation of a line intersecting the given lines (i) and (ii) will be
Page 5
Edurev123
3. Plane and its Properties
3.1 Find the equations of the straight line through the point (?? ,?? ,?? ) to intersect the
straight line
?? +?? =?? +?? =?? (?? -?? )
and parallel to the plane ?? ?? +?? +?? ?? =?? .
(2011 : 10 Marks)
Solution:
Let the required line intersects the given line
?? +?? =?? +1=2(?? -2) (??)
at (?? 1
,?? 1
,?? 1
)
? The equations of the required line passing through (2,1,2) and (?? 1
,?? 1
,?? 1
) are
?? -3
?? 1
-3
=
?? -1
?? 1
-1
=
?? -2
?? 1
-1
(???? )
The direction ratios of the line (ii) are ?? 1
-3,?? 1
-1,?? 1
-1.
Because the line (ii) is parallel to the plane 4?? +?? +5?? =0, therefore the normal to the
plane with direction ratios 4,1,5 is perpendicular to the line (ii).
? 4(?? 1
-3)+1(?? 1
-1)+5(?? 1
-1)=0
or 4?? 1
+?? 2
+5?? 2
-23=0 (?????? )
Because the point (?? 1
,?? 1
,?? 1
) lies on (i),
? ?? 1
+4=?? 1
+1=2(?? 1
-2)
? ?? 1
=2?? 1
-8,?? 1
=2?? 1
-5 (iv)
? from (iv),we get
4(2?? 1
-8)+2?? 1
-5+5?? 1
-23 =0
? ?? 1
=4
? from (iv), ?? 1
=0,?? 1
=3
? from (ii), the equations of the required line are :
?? -3
-3
=
?? -1
2
=
?? 2
-2
2
3.2 Find the equation of the plane which passes through the points (?? ,?? ,?? ) and
(?? ,?? ,-?? ) and is parallel to the line joining the points (-?? ,?? ,-?? ),(?? ,-?? ,?? ) . Find also
the distance between the line and the plane.
(2013 : 10 Marks)
Solution:
The general equation of plane through (0,1,1)
??(?? -0)+?? (?? -1)+?? (?? -1)=0
???? +?? (?? -1)+?? (?? -1)=0
(??)
(2,0,-1) lies on this plane.
? 2?? -?? -2?? =0
Direction ratios of line passing through (-1,1,-2),(3,-2,4) .
?? 1
=3-1=4,?? ?? =(-2-1)=-3;?? 1
=(4-(-2))=6
? Direction ratios are 4::-3::6
This is parallel to plane.
? 4?? -3?? +6?? =0
From (ii) and (iii)
?? -12
=
?? -20
=
?? -2
??? :?? :?? ::6:10:1
? Equation of plane is
6?? +10(?? -1)+(?? -1) =0
? 6?? +10?? +?? -11 =0
Equation of plane parallel to this plane and passing through line
6(?? +1)+10(?? -1)+1(?? +2)=0
6?? +10?? +?? -2=0
? distance between line and plane
= distance between two plane
=
|?? 1
-?? 2
|
v
??˜2
+?? 2
+?? 2
=
9
v6
2
+10
2
+?? 2
This must be on the plane for the intersection of this sphere to be a great circle.
? 5(
5?? -3
2
)-(4-2?? )+2(4?? -2)+7=0
? 45?? =17??? =
17
45
? The given sphere is
?? 2
+?? 2
+?? 2
-
10
9
?? +
146
45
?? -
22
45
?? -
106
45
=0
? 45(?? 2
+?? 2
+?? 2
)-50?? +146?? -22?? -106=0
3.3 Obtain the equation of the plane passing through the points (?? ,?? ,?? ) and
(?? ,-?? ,?? ) parallei to ?? -axis.
(2015: 6 Marks)
Solution:
The equation of any plane through (2,3,1) is
?? (?? -2)+?? (?? -3)+?? (?? -1)=0 (??)
It passes through (4,-5,3)
? ?? (4-2)+?? (-5-3)+?? (3-1)=0
i.e., ?? -4?? +?? =0 (???? )
If the plane (i) is parallel to ?? -axis, then it is perpendicular to ???? -plar 3, i.e., ?? =0, i.e.,
? 1?? +0?? +0?? =0
? from (ii), 1?? +0?? +0?? =0??? =0
?
?? 0
=
?? 1
=
?? 4
Hence, (i) becomes 0+1(?? -3)+4(?? -1)=0
?? +4?? -7=0
3.4 Find the surface generated by a line which intersects the lines ?? =?? =?? ,?? +
?? ?? =?? =?? +?? and parallel to the plane ?? +?? =?? .
(2016 : 10 Marks)
Solution:
Topic : Equation of a straight line intersecting two given lines.
Given lines are :
?? -?? =0=?? -?? (??)
?? +3?? -?? =0=?? +?? -?? (???? )
Hence, the equation of a line intersecting the given lines (i) and (ii) will be
(?? -?? )+?? (?? -?? )=0 (??)*
and (?? +3?? -?? )+?? (?? +?? -?? )=0
? ?? +???? -(?? +???? )=0 (???? )*
and ?? +???? +(3+?? )?? -(?? +???? )=0 (?????? )
Line (iii) is parallel to the plane ?? +?? =0 (???? )
If direction ratio's of line (iii) are ??,?? ,?? , then
1
3+?? -????
=
?? ?? -0
=
?? 0-1
? (iv) ? 1·(3+?? -???? )+1·?? +0·(-1)=0
3+?? +?? -???? =0 (?? )
The required locus of the line is obtained by eliminating ?? and ?? between (??)
*
, (ii)
*
and
(?? ) .
3-
?? -?? ?? -?? -
?? +3?? -?? ?? +?? -?? -
?? -?? ?? -?? ·
?? +3?? -?? ?? +?? -?? =0
Solving and simplifying : (?? +?? )(?? +?? )=2?? (?? +?? )
3.5 Find the projection of straight line
?? -?? ?? =
?? -?? ?? =
?? +?? -?? on the plane ?? +?? +?? ?? =?? .
(2018: 10 marks)
Solution:
Given line is
?? -1
2
=
?? -1
3
=
?? +1
-1
=??
Let this line meets given plane at (2?? +1,3?? +1,-?? -1) .
The point lies on given plane, i.e.,
2?? +?? +3?? +1-2?? -?? =6
? ?? =2
? The point is (5,7,-3) .
Let equation of line of projection is
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