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UPSC CDS Math Basic operation and Factorisation Questions
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© examsnet.com
Question : 3
Total: 77
If
x
=
1
+
√
3
2
and
y
=
x
3
,then y satisfies which one of the following equations ?
[2019 CDS-II]
8
y
2
−
20
y
−
1
=
0
8
y
2
+
20
y
−
1
=
0
8
y
2
+
20
y
+
1
=
0
8
y
2
−
20
y
+
1
=
0
Validate
Solution:
y
=
x
3
⇒
x
=
3
√
y
⇒
3
√
y
=
√
3
+
1
2
⇒
2
3
√
y
=
√
3
+
1
⇒
8
y
=
3
√
3
+
9
+
3
√
3
+
1
(cutting)
⇒
(
4
y
−
5
)
2
=
27
⇒
16
y
2
−
40
y
+
25
=
27
⇒
16
y
2
−
40
y
−
2
=
0
⇒
8
y
2
−
20
y
−
1
=
0
© examsnet.com
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