Question

Find the sum of the geometrical equation -3 , 18 , -108 , if there are 7 terms

Answer 1

Hello from MrBillDoesMath!

Answer:

Sum = -119,962                    ( I hope!)

Discussion:

Let's determine the pattern.

first term:  -3

2nd term:  (-3) * (-6) = 18                     (multiply first term by -6)

3rd  term:  (18) * (-6) = -108                 (multiply 2nd term by -6)

4th term :  (-108)*(-6) = 648                ( etc)

5th term :  (648)*(-6) =  -3888            (etc)

6th term :  (-3888)*(-6) = 23328         (etc)

7th term:   (23328)*(-6) =  -139967     (etc)

Sum = -119,962

A simpler way to do this is to use the formula for the sum of a geometric series to n terms. The series is

-3  Sum ( -6)^n                n = 0, 1, 2, 3, 4, 5, 6

Regards,  

MrB

P.S.  I'll be on vacation from Friday, Dec 22 to Jan 2, 2019. Have a Great New Year!