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Byju's Answer
Standard VIII
Mathematics
Factorisation by Common Factors
Find the valu...
Question
Find the value of the expression
2
x
3
+
2
x
2
−
7
x
+
72
when
x
=
−
1
+
√
15
2
Open in App
Solution
It is given that
x
=
−
1
+
√
15
2
which can be solved as shown below:
x
=
−
1
+
√
15
2
⇒
2
x
=
−
1
+
√
15
⇒
2
x
+
1
=
√
15
⇒
(
2
x
+
1
)
2
=
(
√
15
)
2
⇒
(
2
x
)
2
+
1
2
+
(
2
×
2
x
×
1
)
=
15
(
∵
(
a
+
b
)
2
=
a
2
+
b
2
+
2
a
b
)
⇒
4
x
2
+
1
+
4
x
=
15
⇒
4
x
2
+
4
x
−
14
=
0
⇒
2
(
2
x
2
+
2
x
−
7
)
=
0
⇒
2
x
2
+
2
x
−
7
=
0
.
.
.
.
(
1
)
Now, consider the given polynomial
2
x
3
+
2
x
2
−
7
x
+
72
as follows:
2
x
3
+
2
x
2
−
7
x
+
72
=
x
(
2
x
2
+
2
x
−
7
)
+
72
=
x
(
0
)
+
72
(
F
r
o
m
e
q
n
1
)
=
(
x
×
0
)
+
72
=
0
+
72
=
72
Hence,
2
x
3
+
2
x
2
−
7
x
+
72
=
72
when
x
=
−
1
+
√
15
2
.
Suggest Corrections
2
Similar questions
Q.
Find the value of the expression
2
x
3
+
2
x
2
−
7
x
+
72
where
x
=
3
+
5
√
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2
.
Q.
If
x
=
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+
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i
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, find the value of
2
x
3
+
2
x
2
−
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x
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.
Q.
When
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√
−
1
2
, find the value of
2
x
3
+
2
x
2
−
7
x
+
72
; and show that it will be unaltered if
3
−
5
√
−
1
2
be substituted for
x
.
Q.
If
2
x
=
3
+
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i
, then the value of
2
x
3
+
2
x
2
−
7
x
+
72
is
Q.
Find the limits of the following expression
1
−
x
2
2
x
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÷
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,
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)
when
x
=
0
.
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