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What is the sum of the first seven terms of the geometric series where a1 = 10 and r = −2?
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What is the sum of the first seven terms of the geometric series where a1 = 10 and r = −2?

User Ralph Tandetzky
by
3.2k points

1 Answer

7 votes

Answer:

  • 430

Explanation:

Given GP with:

  • a1 = 10
  • r = -2

To find

  • S7

Solution

Formula of sum:

  • Sn = a1(r^n - 1)/(r - 1)

Substitute values:

  • S7 = 10((-2)^7 - 1)/(-2 - 1) = 10(- 128 - 1)/(-3) = 10*129/3 = 430
User Daveaspinall
by
3.3k points
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