taufajane6245 taufajane6245

23-06-2018
Mathematics

contestada

Math help! PLEAAASSEE

Evaluate the summation of 2 times negative 2 to the n minus 1 power, from n equals 1 to 7.

−256
−84
84
86

Respuesta :

sqdancefan sqdancefan

23-06-2018

The formula for the sum of a geometric sequence with first term "a", common ratio "r" and number of terms "n" is
S = a·(r^n -1)/(r -1)
You have a=2, r=-2, n=7, so the sum is
S = 2·((-2)^7 -1)/(-2 -1)
= 2(-128-1)/(-3)
= 2(43) = 86

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