asked 7.7k views
2 votes
Find the geometric sum 4 + 12 + 36 + . . . + 236,196.

1,062,880

4,194,303

354,292

118,096

1 Answer

4 votes
The answer choices are sufficiently far apart that you can work this backward. The sum will be ...
236,196*(1 + 1/3 + 1/9 + 1/27 + ...)
so a reasonable estimate can be given by an infinite series with a common ratio of 1/3. That sum is
236,196*(1/(1 - 1/3)) = 236,196*(3/2)

Without doing any detailed calculation, you know the best answer choice is ...
354,292


_____
There are log(236196/4)/log(3) + 1 = 11 terms* in the series, so the sum will be found to be 4(3^11 -1)/(3-1) = 2*(3^11-1) = 354,292.

Using the above approach (working backward from the last term), the sum will be 236,196*(1-(1/3)^11)/(1-(1/3)) = 236,196*1.49999153246 = 354,292

___
* If you just compute log(236196/4)/log(3) = 10 terms, then your sum comes out 118,096--a tempting choice. However, you must realize that the last term is larger than this, so this will not be the sum. (In fact, the sum is this value added to the last term.)
answered
User Apangin
by
8.1k points

Related questions

1 answer
3 votes
75.0k views
2 answers
5 votes
113k views
Welcome to Qamnty — a place to ask, share, and grow together. Join our community and get real answers from real people.