The sum of the first n term of a geometric series of the first l term is 127 and the sum of their reciprocal is 127/64.the first term is 1, find n,l and the common ratio

Answer 1

Let the common ratio be r.
The sum of the first n terms of the geometric series is given by the formula: Sn = a(1 - r^n)/(1 - r), where a is the first term.
Substituting a = 1, we have: Sn = (1 - r^n)/(1 - r) = 127 .......(1)
Given that the sum of the reciprocals of the first l terms is 127/64.
The sum of the first l terms of the geometric series is given by the formula: Sl = a(r^l - 1)/(r - 1)
Substituting a = 1, we have: Sl = (r^l - 1)/(r - 1) = 127/64 .......(2)

Dividing equation (2) by equation (1), we get:
[(r^l - 1)/(r - 1)]/[(1 - r^n)/(1 - r)] = (127/64)/127
Simplifying, we have : (r^l - 1)(1 - r)/[(1 - r^n)(r - 1)] = 1/64
(r^l - 1)/(1 - r^n) = 1/64 .......(3)

Since 1 is the first term of the series, the sum of the reciprocals of the first l terms is also the sum of the reciprocals of the first l - 1 terms plus 1/l. Hence, we have the equation:
Sl = Sl-1 + 1/l
(a(r^l - 1)/(r - 1)) = (a(r^(l-1) - 1)/(r - 1)) + 1/l .......(4)
Substituting a = 1 in equation (4), and simplifying, we get:
(r^l - 1)/(r - 1) = (r^(l-1) - 1)/(r - 1) + 1/l
(r^l - 1)/(r - 1) = [(r^(l-1) - 1)+ (r - 1)]/(r - 1)
(r^l - 1)/(r - 1) = (r^l - 1)/(r - 1)
Hence, equation (4) holds true.

Equating the right-hand side of equation (4) with equation (3), we get:
[(r^l - 1)/(1 - r^n)] = 1/64
Cross-multiplying, we have: 64(r^l - 1) = (1 - r^n)
64r^l - 64 = 1 - r^n
64r^l + r^n = 65 .......(5)

Simplifying equation (5), we see that 64r^l is an integer, and r^n is a positive real number between 0 and 1.
Since r^n is a positive real number between 0 and 1, there are only a few possible values for n. To find these values, we can substitute r from equation (1) into equation (5) and examine the possible values of n for each value of l.

Let's try some values of l and find the corresponding values of n:
For l = 2, equation (5) becomes:
64r^2 + r^n = 65
Let's substitute some values of r, such as 1/2, 1/3, 1/4, etc. and see if there is an integer solution for n.

l = 2, r = 1/2:
64(1/2)^2 + 1/2^n = 65
16 + 1/2^n = 65
1/2^n = 49
2^n = 1/49
Taking logarithm to the base 2 on both sides, we get:
n = -log2(49) ...........(6)
n = -log(49) / log(2) .......... (7)

Similarly, we can substitute r = 1/3, 1/4, etc. to find the corresponding values of n.

l = 2, r = 1/3:
64(1/3)^2 + 1/3^n = 65
64/9 + 1/3^n = 65
1/3^n = 64/9
3^n = 9/64
Taking logarithm to the base 3 on both sides, we get:
n = log3(9/64) ...........(8)

l = 2, r = 1/4:
64(1/4)^2 + 1/4^n = 65
16/64 + 1/4^n = 65
1/4^n = 64/48
4^n = 48/64
Taking logarithm to the base 4 on both sides, we get:
n = log4(48/64) ...........(9)

By substituting different values of l and r into equation (5) and finding the corresponding values of n using logarithms, we can explore all possible solutions and determine the values of n, l, and r that satisfy the conditions provided.