Posted by melyssa on .
Find a1 in a geometric series for which Sn = 300, r = 3, and n = 4

algebra 34 
Reiny,
S(n) = a( r^n  1)/(r1)
a( (3)^4  1)/(31) = 300
a( 80 )/(4) = 300
20a = 300
a = 15
check:
first 4 terms would be 15 , +45,  135, + 405
and 15 + 45  135 + 405 = 300