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yaroslaw [1]
3 years ago
6

PLEASE NEED HELP ASAP

Mathematics
1 answer:
Vladimir [108]3 years ago
7 0

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)

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