algebra 2
posted by Isabel .
find a1 for given geometric series.
Sn= 405, r=3, n=4
I don't know how to start the problem

use your formula for S(n)
S(n) = a(r^n  1)/(r1)
405 = a(3^4  1)/(31)
405 = a(81  1)/2
405 = 40a
a = 405/40 = 81/8