Hello!
Probably we need to find all the terms of this progression.
Recall that each next term of a geometric progression is obtained by multiplying the previous term by the quotient, denote it as q. Denote the first term as b, then the second is b*q, the third is b*q*q = b*q^2, and the k-th term is b*q^(k-1).
It is also well-known that the sum of n terms of a geometric progression with the first term b and q!=1 is S = b*(q^n-1)/(q-1).
In our case q=2, b is unknown, n = 7 and S = 127. This way we obtain a simple equation for b:
127 = b*(2^7-1)/(2-1) = b*127, because 2^7 = 128 (check by multiplying 2*2*2*2*2*2*2).
Obviously the only solution is b=1, and the entire array is 1, 2, 4, 8, 16, 32, 64.
We could solve this problem without the formula for sum, adding all 7 terms manually:
b+2b+4b+8b+16b+32b+64b = 127, the left side is (1+2+4+8+16+32+64)b = 127b = 127, and again b=1.
Sunday, October 22, 2017
The sum of the first seven terms of a geometric sequence is 127 and the quotient is 2, find the terms of the sequence.
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