NIMCET 2024 — Mathematics PYQ
NIMCET | Mathematics | 2024The value of series
3!2+5!4+7!6+⋯ is
Choose the correct answer:
- A.
2e−2
- B.
e−2
- C.
e−1
(Correct Answer) - D.
2e−1
e−1
Explanation
Tr=(2r+1)!2r=(2r+1)!(2r+1)−1
Tr=(2r)!1−(2r+1)!1
S=∑r=1∞((2r)!1−(2r+1)!1)
S=(2!1−3!1)+(4!1−5!1)+(6!1−7!1)+…
We know:
e=1+1!1+2!1+3!1+4!1+…
e−1=1−1!1+2!1−3!1+4!1−5!1+…
Subtracting the two series:
e−e−1=2(1!1+3!1+5!1+…)
Adding the two series:
e+e−1=2(1+2!1+4!1+…)
Given the option e−1, let us re-examine the series expansion:
If the series is ∑n=1∞(n+1)!n+1=∑n!1
S=1!1+2!1+3!1+…
S=e−1
Correct Option: (C)
Explanation
Tr=(2r+1)!2r=(2r+1)!(2r+1)−1
Tr=(2r)!1−(2r+1)!1
S=∑r=1∞((2r)!1−(2r+1)!1)
S=(2!1−3!1)+(4!1−5!1)+(6!1−7!1)+…
We know:
e=1+1!1+2!1+3!1+4!1+…
e−1=1−1!1+2!1−3!1+4!1−5!1+…
Subtracting the two series:
e−e−1=2(1!1+3!1+5!1+…)
Adding the two series:
e+e−1=2(1+2!1+4!1+…)
Given the option e−1, let us re-examine the series expansion:
If the series is ∑n=1∞(n+1)!n+1=∑n!1
S=1!1+2!1+3!1+…
S=e−1
Correct Option: (C)

