How to solve three number questions?
1 answer:
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By using the general formula for a geometric sequence, we will find that the correct option is c: 86
<h3>Working with geometric sequences:</h3>
The general formula for the nth term in a geometric sequence is:
Where r is the common ratio of the geometric sequence here we know that:
- a₁ = 2
- a₇ = 128
- r is negative.
Replacing the first two in the general formula we get:
Where we took the negative solution for the root. Now that we know the common ratio we can find the other terms:
The sum gives:
2 - 4 + 8 - 16 + 32 - 64 + 128 = 86
So the correct option is c.
If you want to learn more about geometric sequences, you can read: