What is the sum of the geometric sequence -4, 24, -144, ...if there are 7 terms?

Question

anonymous · Answer

-159,924
-26,661
26,661
159,963

radar · Answer

The multiplier is obviously a -6 so since it is only 7 terms I would just add them up.

Positive values:  24     864    31104

Negative vbalues; -4  -144    -5184    -186624        That is 7th.  combine the negatives getting -191956
Combine the positives, getting:  31992

Now combine the positive and negative getting -159964

radar · Answer

Now solve using the method that you were taught, you should get something close.

anonymous · Answer

Why do you combine them if it already says that the 7th term is -186624? If I do the math on paper it says the same thing. I know that's not the answer but why do you have to combine them in the end?

radar · Answer

You combine them because your problem asked for the "sum"   which means you add, watch the signs.  I am sure there is a method where you can do it with a single equation that you learn when learning "sequences"

radar · Answer

I would have to review my sequences to get that.  Maybe you have it handy.

anonymous · Answer

Oh, okay. Well thank you very much for your help :)

radar · Answer

You're welcome and good luck with your studies.

radar · Answer

@ebonyninja I found the formulae for partial sums of a geometric sequence with a -r
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