Answer:
121.5
Step-by-step explanation:
a geometric series : a1 + a1×r + a1×r^2 + a1×r^3 + ...
so, a1 = 50
a2 = a1×r = 30
=> 50×r = 30
=> r = 30/50 = 3/5
a3 = a2×r = a1×r^2 = 50×9/25 = 18
S7 = (50 - 50×(3/5)^7) / (1 - 3/5) =
= (50 - 50×(2187/78125)) / (2/5) =
= 5×(50 - 50×(2187/78125)) / 2 =
= 5×(25 - 25×(2187/78125)) = 125×(1 - 2187/78125) =
= 125×(78125-2187)/78125 = 125×75938/78125 =
= 75938 / 625 =
121.5 (121.5008)
