3 years ago

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

1 answer:

const2013 [10]3 years ago

4 0

$S_7=-4+24-144+864-5184+31104-186624$
$S_7=-4(1-6+36-216+1296-7776+46656)$
$(-6)S_7=-4(-6+36-216+1296-7776+46656-279936)$
$S_7-(-6)S_7=-4(1+279936)$
$S_7=-\dfrac47\times279937=-159964$

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