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Julli [10]
3 years ago
6

what is the sum of a 7-term geometric series if the first term is negative 11 the last turn is -45056 and the common ratio is -4

Mathematics
1 answer:
patriot [66]3 years ago
3 0
For a geometric sequence with common ratio r, we have

a_n=a_{n-1}r


that is, the n-th term in the sequence is the product of the previous term and the common ratio r. So

a_n=a_{n-1}r=a_{n-2}r^2=\cdots=a_1r^{n-1}

Then the sum of the first 7 terms is

S_7=\displaystyle\sum_{n=1}^7a_n=a_1+a_2+\cdots+a_7
\implies S_7=a_1+a_1r+\cdots+a_1r^6

Notice that

S_7r=a_1r+a_1r^2+\cdots+a_1r^7

so we can subtract this modified sum from S_7 to get

S_7-S_7r=S_7(1-r)=a_1-a_1r^7\implies S_7=a_1\dfrac{1-r^7}{1-r}

We're told that a_1=-11 and r=-4, so the sum of the first 7 terms is

S_7=(-11)\dfrac{1-(-4)^7}{1-(-4)}=144,188
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