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What is the sum of the first five terms of the geometric series 1 − 3 + 9 −...
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What is the sum of the first five terms of the geometric series 1 − 3 + 9 −...

User Joelpet
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\bf 1~~,~~\stackrel{-3\cdot 1}{-3}~~,~~\stackrel{-3\cdot -3}{9}~~,~~\stackrel{-3\cdot 9}{-27}~~,~~\stackrel{-3\cdot -27}{81}

notice, the "common ratio" is -3, thus their sum is just

1 + (-3) + 9 + (-27) + 81.
User Laokoon
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