Question

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

Answer 1

• 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