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यदि `S_(1),S_(2),....S_(n)`उन गुणोत्तर श्रेणियों के अनन्त पदों के योग है जिनके प्रथम पद क्रमशः 1,2,3,...n तथा सार्वअनुपात क्रमशः `(1)/(2),(1)/(3),(1)/(4),....(1)/(n+1)` है, तो `S_(1)^(2)+S_(2)^(2)+....S_(2n-1)^(2)` का मान ज्ञात कीजिए | |
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Answer» Correct Answer - `(1)/(3)n(2n+1)(4n+1)-1` `S_(1)=(a)/(1-r)=(1)/(1-(1)/(2))=2,S_(2)=(2)/(1-1//3)=3,S_(3)=(3)/(1-(1)/(4))=4` `S_(n)=(n)/(1-(1)/(n+1))=n+1 rArr S_(2n-1)=2n-1+1=2n` `:. S_(1)^(2)+S_(2)^(2)+S_(3)^(2)+….+S_(2n-1)^(2)=2^(2)+3^(2)+4^(2)+…(2n)^(2)` `=1^(2)+2^(2)+3^(2)+4^(2)+....+(2n)^(2)-1^(2)=Sigma(2n)^(2)-1` `=(2n(2n-1)(4n+1))/(6)-1=(n(2n+1)(4n+1))/(3)-1` |
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