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Byju's Answer
Standard XII
Mathematics
Collinear Vectors
Prove that ...
Question
Prove that
A
,
B
,
C
are position vectors and
→
a
,
→
b
and
→
c
respectively are collinear if and only if
(
→
a
×
→
b
)
+
(
→
b
×
→
c
)
+
(
→
c
×
→
a
)
=
→
0
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Solution
If
A
,
B
,
C
are collinear.
→
A
B
and
→
B
C
are parallel.
⇒
→
A
B
×
→
B
C
=
→
0
⇒
(
→
b
−
→
a
)
×
(
→
c
−
→
b
)
=
→
0
⇒
(
→
b
−
→
a
)
×
→
c
−
(
→
b
−
→
a
)
×
→
b
=
→
0
⇒
(
→
b
×
→
c
)
−
(
→
a
×
→
c
)
−
(
→
a
×
→
a
)
+
(
→
a
×
→
b
)
=
0
But
→
b
×
→
b
=
0
∴
(
→
a
×
→
b
)
+
(
→
b
×
→
c
)
+
(
→
c
×
→
a
)
=
0
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1
Similar questions
Q.
Prove that points
A
,
B
,
C
having positions vectors
→
a
,
→
b
,
→
c
are collinear, if
[
→
b
×
→
c
+
→
c
×
→
a
+
→
a
×
→
b
]
=
→
0
Q.
Position vectors of
A
,
B
,
C
are given by
→
a
,
→
b
,
→
c
and they are collinear, prove that
(
→
a
×
→
b
)
+
(
→
b
×
→
c
)
+
(
→
c
×
→
a
)
=
0
.
Q.
If
a
,
b
and
c
are vectors such that
→
a
+
→
b
+
→
c
=
→
0
and
|
→
a
|
=
7
,
|
→
b
|
=
5
and
|
→
c
|
=
3
, then angle between
→
b
and
→
c
is
Q.
For any three vectors
→
a
,
→
b
and
→
c
, prove that
[
→
a
+
→
b
,
→
b
+
→
c
,
→
c
+
→
a
]
=
2
[
→
a
→
b
→
c
]
. Hence prove that the vectors
→
a
+
→
b
,
→
b
+
→
c
,
→
c
+
→
a
are coplanar. If and only if
→
a
,
→
b
,
→
c
are coplanar.
Q.
If A, B, C are three non-collinear points with position vectors
→
a
,
→
b
,
→
c
, respectively, then show that the length of the perpendicular from C on AB is
|
(
→
a
×
→
b
)
+
(
→
b
×
→
c
)
+
(
→
c
×
→
a
)
|
|
→
b
−
→
a
|
.
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