For the sequence -3, -12, -48, … how do you find the sum of the first 12 terms?

1 Answer
Roella W.
Mar 7, 2016

Use the formula sum_0^n = (a(1-r^n))/(1-r) where a is the first term, r is the common ratio and n is the number of terms.

Explanation:

The sequence here is -3, (-3*4), (-3*4*4),......

So the nth term is (-3)*4^(n-1)

The first term is -3 and the common ratio is 4.

sum_1^12 = ((-3)(1-4^12))/(1-4) =((-3)(-16777215))/(-3)

=-16777215

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