Q3 of 8 Page 180

The sum of the first 20 terms of a G.P is 244 times the sum of its first 10 terms. Find the common ratio.


Where a = first term,


r = common ratio


n = number of terms


ATQ,



r20 – 1 = 244(r10 – 1)


r20 – 244r10 + 243 = 0


r20 – 243r10 – r10 + 243 = 0


r10(r10 – 1) – 243(r10 – 1 )= 0


⇒ r = √3 (r = 1 is not valid)


So, the common ratio is √3


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