## anonymous one year ago Find S20 for the geometric series 4 + 12 + 36 + 108 + … 5,034,043,187 5,123,498,245` 6,973,568,800 7,321,089,348

1. rishavraj

for geometric series $S_n = a_1 \times (\frac{ 1 - r^n }{ 1 - r } )$

2. anonymous

so what does that mean

3. rishavraj

a_1 is first term i.e 4 r is common ratio i.e 3

4. rishavraj

so for S_20 $S_{20} = 4 \times \frac{ 1 - 3^{20} }{ 1 - 3 }$

5. mathstudent55

Find the common ratio, r, by dividing a term by its previous term. Then substitute 4 for a1, the value you find for r for r, and 20 for n in the formula @rishavraj gave you above.

6. anonymous

thank you!!