What is the sum of the first 7 terms of the sequence $10xx(-2)^(i-1)$ ?

Question

2258 views around the world

You can reuse this answer
Creative Commons License

Answer 1 · 2016-07-02T05:20:00

430

The sum of the first n terms of a geometric series with first term $a_1$ and common ratio r is given by:

$sum_(i=1)^n = [a_1(1-r^n)]/(1-r)$

In this example, $n=7, a_1 = 10$ and $r=-2$

Hence, $sum_(i=1)^7 = (10*(1-(-2)^7))/(1-(-2))$

$= (10*(1-(-128)))/3$

$=1290/3 = 430$

What is the sum of the first 7 terms of the sequence 10xx(-2)^(i-1)10xx(-2)^(i-1) ?