Spreadsheets: Excel and LibreOffice Calc: the sum of a geometric series

The tutor shows the seriessum() function, found on both Excel and LibreOffice Calc.

A geometric series has the form

Sn = cr0 + cr1 + … + crn-1

An example is

3 + 3*4 + 3*42 + … + 3*45

which might also be written

3 + 12 + 48 + 192 + 768 + 3072

With Excel or LibreOffice Calc, the function to evaluate that sum is =seriessum(a,b,c,d), where

  • a is the number whose exponent increases each term (4, in the example just above)
  • b is the initial exponent of a (0, in the example just above)
  • c is the increment by which the exponent increases each term (1, in the example just above)
  • d is an array of the coefficients of the terms. In the example just above, the coefficient is always 3. You’d need to enter {3,3,3,3,3,3} for d: one 3 for each term.

So, to evaluate 3 + 12 + 48 + 192 + 768 + 3072, you might enter

=seriessum(4,0,1,{3,3,3,3,3,3})

Hopefully you receive the answer 4095:)

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.

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