Page 1
Edurev123
5. Application to Geometry
5.1 A curve in space is defined by the vector equation ?? =?? 2
??ˆ+2?? ??ˆ-?? 3
??ˆ
. Determine the
angle between the tangents to this curve at the ?? =+1 and ?? =-1.
(2013 : 10 Marks)
Solution:
?? =?? 2
??ˆ+2?? ??ˆ-?? 3
??ˆ
????
????
=2?? ??ˆ+2??ˆ-3?? 2
??ˆ
????
????
=|
?? ??
????
|=v4?? 2
+4+9?? 4
??ˆ
=
?? ??
????
=
?? ??
????
????
????
=
2+??ˆ+2??ˆ-3?? 2
??ˆ
v4?? 2
+4+9?? 4
where ??ˆ
is the direction of the tangent
??ˆ|
?? =1
=
2??ˆ+2??ˆ-3??ˆ
v17
??ˆ|
?? =-1
=
-2??ˆ+2??ˆ-3??ˆ
v17
Let ?? be the angle
Then
cos ?? =??ˆ
1
·??ˆ
2
=
1
17
(2??ˆ+2??ˆ-3??ˆ
)·(-2??ˆ+2??ˆ-3??ˆ
)=
-4+4+9
17
=
9
17
? ?? =cos
-1
9
17
5.2 Find the angle between the surfaces ?? ?? +?? ?? +?? ?? -?? =?? and ?? =?? ?? +?? ?? -?? at
(?? ,-?? ,?? ) .
(2015 : 10 Marks)
Solution:
Angle between two surfaces at a point is the angle between the normals to the surfaces
at that point.
Let
Page 2
Edurev123
5. Application to Geometry
5.1 A curve in space is defined by the vector equation ?? =?? 2
??ˆ+2?? ??ˆ-?? 3
??ˆ
. Determine the
angle between the tangents to this curve at the ?? =+1 and ?? =-1.
(2013 : 10 Marks)
Solution:
?? =?? 2
??ˆ+2?? ??ˆ-?? 3
??ˆ
????
????
=2?? ??ˆ+2??ˆ-3?? 2
??ˆ
????
????
=|
?? ??
????
|=v4?? 2
+4+9?? 4
??ˆ
=
?? ??
????
=
?? ??
????
????
????
=
2+??ˆ+2??ˆ-3?? 2
??ˆ
v4?? 2
+4+9?? 4
where ??ˆ
is the direction of the tangent
??ˆ|
?? =1
=
2??ˆ+2??ˆ-3??ˆ
v17
??ˆ|
?? =-1
=
-2??ˆ+2??ˆ-3??ˆ
v17
Let ?? be the angle
Then
cos ?? =??ˆ
1
·??ˆ
2
=
1
17
(2??ˆ+2??ˆ-3??ˆ
)·(-2??ˆ+2??ˆ-3??ˆ
)=
-4+4+9
17
=
9
17
? ?? =cos
-1
9
17
5.2 Find the angle between the surfaces ?? ?? +?? ?? +?? ?? -?? =?? and ?? =?? ?? +?? ?? -?? at
(?? ,-?? ,?? ) .
(2015 : 10 Marks)
Solution:
Angle between two surfaces at a point is the angle between the normals to the surfaces
at that point.
Let
?? 1
=?? 2
+?? 2
+?? 2
and
?? 2
=?? 2
+?? 2
-??
Then grad (?? 1
)=2???? +2???? +2????
grad (?? 2
)=2???? +2???? -??
?????? ?? 1
=grad ?? 1
at point (2,-1,2)=4?? -2?? +4??
?? 2
=grad ?? 2
at point (2,-1,2)=4?? -2?? -4??
The vectors ?? 1
and ?? 2
are along normals to the two surfaces at the point (2,-1,2) . If ?? is
the angle between these two vectors then,
cos ?? =
?? 1
·?? 2
|?? 1
||?? 2
|
=
16+4-4
v16+4+16v16+4+1
=
16
6v21
?? =cos
-1
8
3v21
5.3 Find the values of ?? and ?? so that the surfaces ?? ?? ?? -?????? =(?? +?? )?? and ?? ?? ?? ?? +
?? ?? =?? may intersect orthogonally at (?? ,-?? ,?? ) .
(2015 : 12 Marks)
Solution:
Let
?? 1
=?? ?? 2
-(?? +2)?? -?????? ?? 2
=4?? 2
?? +?? 3
grad ?? 1
=??? 1
=(2???? -?? -2)?? -?????? -?????? ??? 2
=8?????? +4?? 2
?? +3?? 2
??
At point (1,-1,2)
?? 1
=(?? -2)?? -2???? +????
?? 2
=-8?? +4?? +12?? ?? 1
·?? 2
=0
-8(?? -2)-8?? +12?? =0
(intersect orthogonally)
-8?? +4?? =-16?2?? -?? =4 (??)
Point (1,-1,2) lies on surface ?? ?? 2
-?????? =(?? +2)??
Page 3
Edurev123
5. Application to Geometry
5.1 A curve in space is defined by the vector equation ?? =?? 2
??ˆ+2?? ??ˆ-?? 3
??ˆ
. Determine the
angle between the tangents to this curve at the ?? =+1 and ?? =-1.
(2013 : 10 Marks)
Solution:
?? =?? 2
??ˆ+2?? ??ˆ-?? 3
??ˆ
????
????
=2?? ??ˆ+2??ˆ-3?? 2
??ˆ
????
????
=|
?? ??
????
|=v4?? 2
+4+9?? 4
??ˆ
=
?? ??
????
=
?? ??
????
????
????
=
2+??ˆ+2??ˆ-3?? 2
??ˆ
v4?? 2
+4+9?? 4
where ??ˆ
is the direction of the tangent
??ˆ|
?? =1
=
2??ˆ+2??ˆ-3??ˆ
v17
??ˆ|
?? =-1
=
-2??ˆ+2??ˆ-3??ˆ
v17
Let ?? be the angle
Then
cos ?? =??ˆ
1
·??ˆ
2
=
1
17
(2??ˆ+2??ˆ-3??ˆ
)·(-2??ˆ+2??ˆ-3??ˆ
)=
-4+4+9
17
=
9
17
? ?? =cos
-1
9
17
5.2 Find the angle between the surfaces ?? ?? +?? ?? +?? ?? -?? =?? and ?? =?? ?? +?? ?? -?? at
(?? ,-?? ,?? ) .
(2015 : 10 Marks)
Solution:
Angle between two surfaces at a point is the angle between the normals to the surfaces
at that point.
Let
?? 1
=?? 2
+?? 2
+?? 2
and
?? 2
=?? 2
+?? 2
-??
Then grad (?? 1
)=2???? +2???? +2????
grad (?? 2
)=2???? +2???? -??
?????? ?? 1
=grad ?? 1
at point (2,-1,2)=4?? -2?? +4??
?? 2
=grad ?? 2
at point (2,-1,2)=4?? -2?? -4??
The vectors ?? 1
and ?? 2
are along normals to the two surfaces at the point (2,-1,2) . If ?? is
the angle between these two vectors then,
cos ?? =
?? 1
·?? 2
|?? 1
||?? 2
|
=
16+4-4
v16+4+16v16+4+1
=
16
6v21
?? =cos
-1
8
3v21
5.3 Find the values of ?? and ?? so that the surfaces ?? ?? ?? -?????? =(?? +?? )?? and ?? ?? ?? ?? +
?? ?? =?? may intersect orthogonally at (?? ,-?? ,?? ) .
(2015 : 12 Marks)
Solution:
Let
?? 1
=?? ?? 2
-(?? +2)?? -?????? ?? 2
=4?? 2
?? +?? 3
grad ?? 1
=??? 1
=(2???? -?? -2)?? -?????? -?????? ??? 2
=8?????? +4?? 2
?? +3?? 2
??
At point (1,-1,2)
?? 1
=(?? -2)?? -2???? +????
?? 2
=-8?? +4?? +12?? ?? 1
·?? 2
=0
-8(?? -2)-8?? +12?? =0
(intersect orthogonally)
-8?? +4?? =-16?2?? -?? =4 (??)
Point (1,-1,2) lies on surface ?? ?? 2
-?????? =(?? +2)??
? ?? +2?? =(?? +2)??? =1
? 2?? -1=4??? =
5
2
5.4 Find the curvature vector and its magnitude at any point ??? =(?? ) of the curve
??? =(?? ?????? ?? ,?? ?????? ?? , a). Show that the locus of the feet of the perpendicular from the
origin to the tangent is a curve that completely lies on the hyperboloid ?? ?? +?? ?? -
?? ?? =?? ?? .
(2017 : 16 Marks)
Solution:
?? =?? cos ???? +?? sin ???? +??????
?? ??
????
=-?? sin ???? +?? cos ???? +????
????
????
=|
????
????
|=v2??
?? =
?? ??
????
=
?? ??
????
????
=
1
v2
(-sin ???? +cos ???? +?? )
?? ??
????
=
1
v2
(-cos ?? ??ˆ-sin ?? ??ˆ)
?????????????????? ???????????? , ???
=
?? ??
????
=
?? ?? /????
???? /????
=
1
v2?? ·
1
v2
(-cos ???? -sin ???? )
=
-cos ????
2?? ·
sin ????
2??
Magnitude,
|?? |=
1
2??
Locus of feet of perpendicular from origin to tangent
Page 4
Edurev123
5. Application to Geometry
5.1 A curve in space is defined by the vector equation ?? =?? 2
??ˆ+2?? ??ˆ-?? 3
??ˆ
. Determine the
angle between the tangents to this curve at the ?? =+1 and ?? =-1.
(2013 : 10 Marks)
Solution:
?? =?? 2
??ˆ+2?? ??ˆ-?? 3
??ˆ
????
????
=2?? ??ˆ+2??ˆ-3?? 2
??ˆ
????
????
=|
?? ??
????
|=v4?? 2
+4+9?? 4
??ˆ
=
?? ??
????
=
?? ??
????
????
????
=
2+??ˆ+2??ˆ-3?? 2
??ˆ
v4?? 2
+4+9?? 4
where ??ˆ
is the direction of the tangent
??ˆ|
?? =1
=
2??ˆ+2??ˆ-3??ˆ
v17
??ˆ|
?? =-1
=
-2??ˆ+2??ˆ-3??ˆ
v17
Let ?? be the angle
Then
cos ?? =??ˆ
1
·??ˆ
2
=
1
17
(2??ˆ+2??ˆ-3??ˆ
)·(-2??ˆ+2??ˆ-3??ˆ
)=
-4+4+9
17
=
9
17
? ?? =cos
-1
9
17
5.2 Find the angle between the surfaces ?? ?? +?? ?? +?? ?? -?? =?? and ?? =?? ?? +?? ?? -?? at
(?? ,-?? ,?? ) .
(2015 : 10 Marks)
Solution:
Angle between two surfaces at a point is the angle between the normals to the surfaces
at that point.
Let
?? 1
=?? 2
+?? 2
+?? 2
and
?? 2
=?? 2
+?? 2
-??
Then grad (?? 1
)=2???? +2???? +2????
grad (?? 2
)=2???? +2???? -??
?????? ?? 1
=grad ?? 1
at point (2,-1,2)=4?? -2?? +4??
?? 2
=grad ?? 2
at point (2,-1,2)=4?? -2?? -4??
The vectors ?? 1
and ?? 2
are along normals to the two surfaces at the point (2,-1,2) . If ?? is
the angle between these two vectors then,
cos ?? =
?? 1
·?? 2
|?? 1
||?? 2
|
=
16+4-4
v16+4+16v16+4+1
=
16
6v21
?? =cos
-1
8
3v21
5.3 Find the values of ?? and ?? so that the surfaces ?? ?? ?? -?????? =(?? +?? )?? and ?? ?? ?? ?? +
?? ?? =?? may intersect orthogonally at (?? ,-?? ,?? ) .
(2015 : 12 Marks)
Solution:
Let
?? 1
=?? ?? 2
-(?? +2)?? -?????? ?? 2
=4?? 2
?? +?? 3
grad ?? 1
=??? 1
=(2???? -?? -2)?? -?????? -?????? ??? 2
=8?????? +4?? 2
?? +3?? 2
??
At point (1,-1,2)
?? 1
=(?? -2)?? -2???? +????
?? 2
=-8?? +4?? +12?? ?? 1
·?? 2
=0
-8(?? -2)-8?? +12?? =0
(intersect orthogonally)
-8?? +4?? =-16?2?? -?? =4 (??)
Point (1,-1,2) lies on surface ?? ?? 2
-?????? =(?? +2)??
? ?? +2?? =(?? +2)??? =1
? 2?? -1=4??? =
5
2
5.4 Find the curvature vector and its magnitude at any point ??? =(?? ) of the curve
??? =(?? ?????? ?? ,?? ?????? ?? , a). Show that the locus of the feet of the perpendicular from the
origin to the tangent is a curve that completely lies on the hyperboloid ?? ?? +?? ?? -
?? ?? =?? ?? .
(2017 : 16 Marks)
Solution:
?? =?? cos ???? +?? sin ???? +??????
?? ??
????
=-?? sin ???? +?? cos ???? +????
????
????
=|
????
????
|=v2??
?? =
?? ??
????
=
?? ??
????
????
=
1
v2
(-sin ???? +cos ???? +?? )
?? ??
????
=
1
v2
(-cos ?? ??ˆ-sin ?? ??ˆ)
?????????????????? ???????????? , ???
=
?? ??
????
=
?? ?? /????
???? /????
=
1
v2?? ·
1
v2
(-cos ???? -sin ???? )
=
-cos ????
2?? ·
sin ????
2??
Magnitude,
|?? |=
1
2??
Locus of feet of perpendicular from origin to tangent
????
?????
=?? -(?? ·?? )??
?? = Foot of perpendicular
?? ·?? =(?? cos ???? +?? sin ???? +?????? )·(-sin ???? +cos ?? +?? )
1
v2
=
????
v2
?? -(?? ·?? )?? =(?? cos ???? +?? sin ???? +?????? )-
????
v2
·
1
v2
(-sin ???? +cos ?? +?? )
=(?? cos ?? +
???? sin ?? 2
)?? +(?? sin ?? -
???? cos ?? 2
)?? +
?? 2
??
Let foot of perpendicular be ?? (?? ,?? ,?? )
?? =?? cos ?? +
???? sin ?? 2
?? =?? sin ?? -
???? cos ?? 2
?? =
????
2
? ?? 2
+?? 2
-?? 2
=?? 2
+
?? 2
?? 2
4
-
?? 2
?? 2
4
=?? 2
?? 2
+?? 2
-?? 2
=?? 2
is the required locus.
5.5 Find the angle between tangent at general point of curve whose equations are
?? =?? ?? ,?? =?? ?? ?? , ?? =?? ?? ?? and the line ?? -?? =?? =?? .
(2018: 10 marks)
Solution:
The radius vector of given curve, ?? can be written as
Page 5
Edurev123
5. Application to Geometry
5.1 A curve in space is defined by the vector equation ?? =?? 2
??ˆ+2?? ??ˆ-?? 3
??ˆ
. Determine the
angle between the tangents to this curve at the ?? =+1 and ?? =-1.
(2013 : 10 Marks)
Solution:
?? =?? 2
??ˆ+2?? ??ˆ-?? 3
??ˆ
????
????
=2?? ??ˆ+2??ˆ-3?? 2
??ˆ
????
????
=|
?? ??
????
|=v4?? 2
+4+9?? 4
??ˆ
=
?? ??
????
=
?? ??
????
????
????
=
2+??ˆ+2??ˆ-3?? 2
??ˆ
v4?? 2
+4+9?? 4
where ??ˆ
is the direction of the tangent
??ˆ|
?? =1
=
2??ˆ+2??ˆ-3??ˆ
v17
??ˆ|
?? =-1
=
-2??ˆ+2??ˆ-3??ˆ
v17
Let ?? be the angle
Then
cos ?? =??ˆ
1
·??ˆ
2
=
1
17
(2??ˆ+2??ˆ-3??ˆ
)·(-2??ˆ+2??ˆ-3??ˆ
)=
-4+4+9
17
=
9
17
? ?? =cos
-1
9
17
5.2 Find the angle between the surfaces ?? ?? +?? ?? +?? ?? -?? =?? and ?? =?? ?? +?? ?? -?? at
(?? ,-?? ,?? ) .
(2015 : 10 Marks)
Solution:
Angle between two surfaces at a point is the angle between the normals to the surfaces
at that point.
Let
?? 1
=?? 2
+?? 2
+?? 2
and
?? 2
=?? 2
+?? 2
-??
Then grad (?? 1
)=2???? +2???? +2????
grad (?? 2
)=2???? +2???? -??
?????? ?? 1
=grad ?? 1
at point (2,-1,2)=4?? -2?? +4??
?? 2
=grad ?? 2
at point (2,-1,2)=4?? -2?? -4??
The vectors ?? 1
and ?? 2
are along normals to the two surfaces at the point (2,-1,2) . If ?? is
the angle between these two vectors then,
cos ?? =
?? 1
·?? 2
|?? 1
||?? 2
|
=
16+4-4
v16+4+16v16+4+1
=
16
6v21
?? =cos
-1
8
3v21
5.3 Find the values of ?? and ?? so that the surfaces ?? ?? ?? -?????? =(?? +?? )?? and ?? ?? ?? ?? +
?? ?? =?? may intersect orthogonally at (?? ,-?? ,?? ) .
(2015 : 12 Marks)
Solution:
Let
?? 1
=?? ?? 2
-(?? +2)?? -?????? ?? 2
=4?? 2
?? +?? 3
grad ?? 1
=??? 1
=(2???? -?? -2)?? -?????? -?????? ??? 2
=8?????? +4?? 2
?? +3?? 2
??
At point (1,-1,2)
?? 1
=(?? -2)?? -2???? +????
?? 2
=-8?? +4?? +12?? ?? 1
·?? 2
=0
-8(?? -2)-8?? +12?? =0
(intersect orthogonally)
-8?? +4?? =-16?2?? -?? =4 (??)
Point (1,-1,2) lies on surface ?? ?? 2
-?????? =(?? +2)??
? ?? +2?? =(?? +2)??? =1
? 2?? -1=4??? =
5
2
5.4 Find the curvature vector and its magnitude at any point ??? =(?? ) of the curve
??? =(?? ?????? ?? ,?? ?????? ?? , a). Show that the locus of the feet of the perpendicular from the
origin to the tangent is a curve that completely lies on the hyperboloid ?? ?? +?? ?? -
?? ?? =?? ?? .
(2017 : 16 Marks)
Solution:
?? =?? cos ???? +?? sin ???? +??????
?? ??
????
=-?? sin ???? +?? cos ???? +????
????
????
=|
????
????
|=v2??
?? =
?? ??
????
=
?? ??
????
????
=
1
v2
(-sin ???? +cos ???? +?? )
?? ??
????
=
1
v2
(-cos ?? ??ˆ-sin ?? ??ˆ)
?????????????????? ???????????? , ???
=
?? ??
????
=
?? ?? /????
???? /????
=
1
v2?? ·
1
v2
(-cos ???? -sin ???? )
=
-cos ????
2?? ·
sin ????
2??
Magnitude,
|?? |=
1
2??
Locus of feet of perpendicular from origin to tangent
????
?????
=?? -(?? ·?? )??
?? = Foot of perpendicular
?? ·?? =(?? cos ???? +?? sin ???? +?????? )·(-sin ???? +cos ?? +?? )
1
v2
=
????
v2
?? -(?? ·?? )?? =(?? cos ???? +?? sin ???? +?????? )-
????
v2
·
1
v2
(-sin ???? +cos ?? +?? )
=(?? cos ?? +
???? sin ?? 2
)?? +(?? sin ?? -
???? cos ?? 2
)?? +
?? 2
??
Let foot of perpendicular be ?? (?? ,?? ,?? )
?? =?? cos ?? +
???? sin ?? 2
?? =?? sin ?? -
???? cos ?? 2
?? =
????
2
? ?? 2
+?? 2
-?? 2
=?? 2
+
?? 2
?? 2
4
-
?? 2
?? 2
4
=?? 2
?? 2
+?? 2
-?? 2
=?? 2
is the required locus.
5.5 Find the angle between tangent at general point of curve whose equations are
?? =?? ?? ,?? =?? ?? ?? , ?? =?? ?? ?? and the line ?? -?? =?? =?? .
(2018: 10 marks)
Solution:
The radius vector of given curve, ?? can be written as
?? =3??ˆ+3?? 2
??ˆ+3?? 3
??ˆ
?? ??
????
=3??ˆ+6??ˆ+9?? 2
??ˆ
????
????
=|
?? ??
????
|=v9+36?? 2
+81?? 4
=3v1+4?? 2
+9?? 4
Tangent vector,
???
=
?? ??
????
=
?? ?? /????
???? /????
=
??
3v1+4?? 2
+9?? 4
×(3??ˆ+6?? ??ˆ)
? ???
=
??ˆ+2?? ??ˆ+3?? 2
??ˆ
v1+4?? 2
+9?? 4
The given line can be written as
?? 1
=
?? 0
=
?? 1
? angle between line and tangent vector, ??
cos ?? =
1×1+2?? ×0+3?? 2
×1
v2·v1+4?? 2
+9?? 4
=
1+3?? 2
v2·v1+4?? 2
+9?? 4
?? =cos
-1
(
1+3?? 2
v2·v1+4?? 2
+9?? 4
)
5.6 Find the circulation of ????
round the curve ?? , where ????
=(?? ?? +?? ?? )??ˆ+(?? ?? -?? ?? )??ˆ
and ?? is the curve ?? =?? ?? from (?? ,?? ) to (?? ,?? ) and the curve ?? ?? =?? from (?? ,?? ) to
(?? ,?? ) .
(2019: 15 Marks)
Solution:
Here the closed curve ' ?? consists of arcs ?????? and ?????? . Let ?? 1
denote the arc ?????? and
?? 2
denote arc ?????? . Along ?? 1
, we have ?? =?? 2
so that ???? =2?????? and ?? varies from 0?1
along ?? 2
, we have ?? =?? 2
so that ???? = 2?????? and ?? varies from 1 to also.
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