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CDS Model Paper 1 Mathematics
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© examsnet.com
Question : 34
Total: 100
If
(
x
2
+
1
x
2
)
=
17
4
, then what is
(
x
3
−
1
x
3
)
equal to?
75
16
63
8
95
8
None of these
Validate
Solution:
(
x
2
+
1
x
2
)
=
17
4
⇒
x
2
+
1
x
2
+
2
−
2
=
17
4
⇒
(
x
−
1
x
)
2
+
2
=
17
4
⇒
(
x
−
1
x
)
2
=
17
4
−
2
⇒
(
x
−
1
x
)
2
=
9
4
⇒
(
x
−
1
x
)
=
3
2
On cubing both sides, we get
(
x
−
1
x
)
3
=
(
3
2
)
3
⇒
x
3
−
1
x
3
−
3
×
1
x
×
(
x
−
1
x
)
=
27
8
⇒
x
3
−
1
x
3
=
27
8
+
3
×
(
3
2
)
⇒
x
3
−
1
x
3
=
27
8
+
9
2
∴
x
3
−
1
x
3
=
63
8
© examsnet.com
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