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Byju's Answer
Standard XII
Mathematics
Condition for Two Lines to Be Parallel
In an equilat...
Question
In an equilateral triangle
A
B
C
,
D
is point on side
B
C
such that
B
D
=
1
3
B
C
. Prove that
9
A
D
2
=
7
A
B
2
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Solution
Let
E
be the midpoint of BC so that
B
E
=
E
C
=
B
C
2
and the length of the sides of the equilateral triangle is
x
. So, we have
A
B
=
B
C
=
C
A
=
x
…
(
1
)
B
E
=
E
C
=
x
2
And it is given that
B
D
=
B
C
3
=
x
3
also
B
E
=
x
2
∴
D
E
=
B
E
−
B
D
=
x
2
−
x
3
=
x
6
Now, In the right angle
△
A
E
D
we have
A
E
=
√
3
x
2
A
D
2
=
A
E
2
+
D
E
2
=
3
x
2
4
+
x
2
36
=
28
x
2
36
36
A
D
2
=
28
x
2
Now put
x
2
=
A
B
2
from equation (1), we get
36
A
D
2
=
28
A
B
2
⟹
9
A
D
2
=
7
A
B
2
[
h
e
n
c
e
p
r
o
v
e
d
]
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Question 15
In an equilateral triangle ABC, D is a point on side BC such that
B
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=
1
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. Prove that
9
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.
Q.
In an equilateral
Δ
A
B
C
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. Prove that
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=
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In an equilateral
△
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,
D
is a point on the side
B
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such that
B
D
=
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B
C
. Prove that
9
(
A
D
)
2
=
7
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A
B
)
2
.
Q.
In an equilateral triangal ABC, D is a point on side BC such that
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D
=
1
3
B
C
.Prove that
9
A
D
2
=
7
A
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Q.
In an equilateral triangle
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