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Byju's Answer
Standard XII
Mathematics
Condition for Concurrency of Three Lines
Prove that a ...
Question
Prove that a necessary and sufficient condition for three vectors
a
→
,
b
→
and
c
→
to be coplanar is that there exist scalars l, m, n not all zero simultaneously such that
l
a
→
+
m
b
→
+
n
c
→
=
0
→
.
Open in App
Solution
Necessary
Condition
:
F
i
r
s
t
l
e
t
a
→
,
b
→
,
c
→
be
three
coplanar
vectors
.
Then
one
of
them
is
expressable
as
a
linear
combination
of
the
other
two
.
Let
c
→
=
x
a
→
+
y
b
→
for
some
scalars
x
,
y
.
Then
,
c
→
=
x
a
→
+
y
b
→
for
some
scalars
x
,
y
⇒
l
a
→
+
m
b
→
+
n
c
→
=
0
,
w
h
e
r
e
l
=
x
,
m
=
y
,
n
=
-
1
Thus
,
i
f
a
→
,
b
→
,
c
→
are
coplanar
vectors
,
then
there
exists
scalars
l
,
m
,
n
s
u
c
h
t
h
a
t
l
a
→
+
m
b
→
+
n
c
→
=
0
w
h
e
r
e
l
,
m
,
n
are
all
non
zero
simultaneously
.
Sufficient
Condition
:
L
e
t
a
→
,
b
→
,
c
→
be
three
vectors
such
that
there
exists
scalars
l
,
m
,
n
n
o
t
a
l
l
z
e
r
o
s
i
m
u
l
a
t
a
n
e
o
u
s
l
y
satisfying
l
a
→
+
m
b
→
+
n
c
→
=
0
→
.
We
have
tp
prove
that
a
→
,
b
→
,
c
→
are
coplanar
vectors
.
Now
,
l
a
→
+
m
b
→
+
n
c
→
=
0
→
⇒
n
c
→
=
-
l
a
→
-
m
b
→
⇒
c
→
=
-
1
n
a
→
+
-
m
n
b
→
⇒
c
→
is
a
linear
combination
of
a
→
a
n
d
b
→
.
Hence
a
→
,
b
→
,
c
→
are
coplanar
vectors
.
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0
Similar questions
Q.
Statement-I :
→
a
×
(
→
b
×
→
c
)
,
→
b
×
(
→
c
×
→
a
)
,
→
c
×
(
→
a
×
→
b
)
are coplanar vectors.
Statement-II: If there exists scalars
l
,
m
,
n
not all zero such that
l
→
a
+
m
→
b
+
n
→
c
=
→
0
, then the vectors
→
a
,
→
b
,
→
c
are coplanar
Q.
If
→
a
,
→
b
,
→
c
are three non-coplanar vectors, and
→
p
,
→
q
,
→
r
are reciprocal vectors to
→
a
,
→
b
,
→
c
respectively, then
(
l
→
a
+
m
→
b
+
n
→
c
)
.
(
l
→
p
+
m
→
q
+
n
→
r
)
is equal to
:
(
where
l
,
m
,
n
are scalars
)
Q.
If a, b, c are three non-coplanar vectors and p, q, r are reciprocal vectors, then
(
l
a
+
m
b
+
n
c
)
.
(
l
p
+
m
q
+
n
r
)
is equal to
Q.
If
x
l
(
m
b
+
n
c
−
l
a
)
=
y
m
(
n
c
+
l
a
−
m
b
)
=
z
n
(
l
a
+
m
b
−
n
c
)
,
prove that
l
x
(
b
y
+
c
z
−
a
x
)
=
m
y
(
c
z
+
a
x
−
b
y
)
=
n
z
(
a
x
+
b
y
−
c
z
)
.
Q.
Prove that the necessary and sufficient condition for an integer n to odd is that
n
2
is odd.
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