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The sum of an infinite geomwtric series is 20 and the sum of the squares of these terms is 100. Find the series.

Answer» Correct Answer - `(8+24/5+72/25+...oo)`
Let a abe the first term and r be the common ratio of the given series.
Then, `a/((1-r))=20 rArr a^(2)/((1-r)^(2))=400` ...(i)
And, `a^(2)/((1-r^(2)))=100` ...(ii)
`:. a^(2)/((1-r)^(2))xx((1-r^(2)))/a^(2)=400/100 rArr (1+r)/(1-r)=4 rArr r=3/5`
`:. a/((1-3/5))=20 rArr a=8`.
Hence, the required series is `(8+24/5+72/25+...oo)`.


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