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Byju's Answer
Standard XII
Mathematics
Integration Using Substitution
If nP2=90, ...
Question
If
n
P
2
=
90
, find '
n
'.
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Solution
Given that:
n
P
2
=
90
⟹
n
!
(
n
−
2
)
!
=
90
....
[
∵
n
P
r
=
n
!
(
n
−
r
)
!
]
⟹
n
(
n
−
1
)
=
90
⟹
n
2
−
n
−
90
=
0
⟹
(
n
+
9
)
(
n
−
10
)
=
0
⟹
n
=
10
or
n
=
−
9
But
n
can't be negative
∴
n
=
10
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