sequences!!!

[12345678]

‘Find the value of N such that

’
(NB this is the first type of this question I have seen)
So I started off by writing out a few terms from each sequence,
(For the 2N one) terms= 1,2,3,4,…2N-1,2N
For the N one) terms = 1,2,3,4…N-1,N
Sn = 1/2n (a + L)
For 2N sequence, a = 1 and L = 2N
Hence Sn = 1/2n (1 + 2N)
For N sequence, a = 1 and L = N
Hence Sn = 1/2n (1 + N)
So: 1/2n + Nn – 1/2n – 1/2Nn = 1256
1/2Nn = 1256
Nn = 2512
Anybody know what to do from here?
(Also, an unrelated question, but what does evaluate: followed by a series with sigma notation mean?)

 

[12345678]

Not sure if the equation came up; its in the form of sigma notation, 2N (one top meaning last term of n) Un meaning function and n=1 meaning 1 is the first value of n) - N (one top) Un and n=1 =1256
s

 

[Deleted member 4993]

‘Find the value of N such that

’.... Your graphics did not post correctly - thus we cannot tell where you were going......
(NB this is the first type of this question I have seen)
So I started off by writing out a few terms from each sequence,
(For the 2N one) terms= 1,2,3,4,…2N-1,2N
For the N one) terms = 1,2,3,4…N-1,N
Sn = 1/2n (a + L)
For 2N sequence, a = 1 and L = 2N
Hence Sn = 1/2n (1 + 2N)
For N sequence, a = 1 and L = N
Hence Sn = 1/2n (1 + N)
So: 1/2n + Nn – 1/2n – 1/2Nn = 1256
1/2Nn = 1256
Nn = 2512
Anybody know what to do from here?
(Also, an unrelated question, but what does evaluate: followed by a series with sigma notation mean?)

In general, sigma like Σ signifies summation.

 

[Deleted member 4993]

Not sure if the equation came up; its in the form of sigma notation, 2N (one top meaning last term of n) Un meaning function and n=1 meaning 1 is the first value of n) - N (one top) Un and n=1 =1256
s

Cannot quite make sense of this.

 

[12345678]

2N N
∑ Un - ∑ Un = 1256 sorry; this is the question

N=1 n=1

 

[12345678]

Never mind, I cannot format the question correctly.

 

[Quaid]

I cannot format the question correctly.

Can you take a picture and attach the image?