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प्रश्न
For a G.P. If t3 = 20 , t6 = 160 , find S7
उत्तर
t3 = 20, t6 = 160
tn = arn–1
∴ t3 = ar3–1 = ar2
∴ ar2 = 20
∴ a =
Also, t6 = ar5
ar5 = 160
∴
∴ r3 =
∴ r = 2
Substituting the value of r in (i) we get
a =
Now, Sn =
∴ S7 =
= 5(128 – 1)
= 635
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