Respuesta :

[tex]\bf 1~~,~~\stackrel{-3\cdot 1}{-3}~~,~~\stackrel{-3\cdot -3}{9}~~,~~\stackrel{-3\cdot 9}{-27}~~,~~\stackrel{-3\cdot -27}{81}[/tex]

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

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

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