Limit as a sum doubt!!

1.)
limn→∞ [1/n+ n2/(n+1)3+ n2/n+23.... 1/8n ]

2.)

lim 1/n[sec2π/4n+ sec22π/4n...sec2nπ/4n]
n→∞

3.)

limn→∞

6 Answers

6
Kalyan IIT-K Beware I'm coming ·

1st one is easy

here is the solution:

limn→∞ [1/n+ n2/(n+1)3+ n2/n+23.... 1/8n ]

am omitting limit but it is there untill i say ...
=1/n[n/n+n3/n+13+.......1/8]

this can be written as

1/n [n3/n3+n3/n+13+.....n3/8n3]

which equals

lim n→∞ Σr=0 to n 1/(1+r/n)3

let x=r/n
therefore dx=1/n

S=∫0 to 1 dx/(1+x)3

=1+x-3+1/-3+1 from 0 to 1

Putting limits we get answer as 3/8....

hope solution is okay!!

1
Avinav Prakash ·

1>the first term caN BE WRITTEN AS
n2(n+0)3 and then integ

n2(n+r)3 from 0 to 1

1
lanuk ·

is ans of 1st 3/8????using the 1st principle of integration....

and ans of 2nd 4/pie????Again using the 1st principle of integration....

6
Kalyan IIT-K Beware I'm coming ·

2nd one same way yarr!! its easy....

lim n→∞ Σ r=1 to n (sec rπ/4n)2

taking 1/n=dx

we get

S= ∫0 to 1 (sec πx /4)2

which comes out to be

4/π[tan π/4x]0 to 1

therefore S= 4/Ï€

This should be the answer!!!

6
Kalyan IIT-K Beware I'm coming ·

yeah @lanuk iam getting same answer as that of urs!!!

1
Euclid ·

@rohna wers the third???

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