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how do you find the sum of 3+15+75+.....+46875 using the Sn formula

Sagot :

3+15+75+.....+46875

a1 = first term = 3

If we multiply the first term by 5, we obtain the second term.

3 x 5 = 15

15x 5 = 75

75x 5= 375

375x5= 1875

1875x5=9375

9375x5= 46875 (7th term) n=7

So,

r= 5

Apply the formula:

an= a1 * r^(n-1)

an= 3 * 5 (n-1)

Sum

Sn = a1 (1 -r^n) / 1- r

S(7) = [3 (1 - 5^7)] / 1-5

S(7) = [3 (1-78,125)] /-4

S(7) = 58,893