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Byju's Answer
Standard XII
Mathematics
Test for Monotonicity in an Interval
The function ...
Question
The function f(x)=
2
x
3
−
9
x
2
+
12
x
+
4
is decreasing in the interval(s)
A
(
−
∞
,
1
)
and
(
2
,
∞
)
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B
(
−
∞
,
1
)
only
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C
(1, 2)
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D
(
2
,
∞
)
only
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Solution
The correct option is
C
(1, 2)
(f(x) =
2
x
3
−
9
x
2
+
12
x
+
4
f'(x) =
6
x
2
−
18
x
+
12
=
6
(
x
2
−
3
x
+
2
)
=
6
(
x
−
1
)
(
x
−
2
)
>
0
f
′
(
x
)
>
0
i
n
(
−
∞
,
−
1
)
∪
(
2
,
∞
)
f
′
(
x
)
<
0
i
n
(
1
,
2
)
Suggest Corrections
0
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Q.
For which interval the given function
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Standard XII Mathematics
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