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Let `S_(n)=sum_(k=1)^(4n)(-1)^((k(k+1))/(2))*k^(2)`, then `S_(n)` can take valueA. 1056B. 1088C. 1120D. 1332 |
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Answer» Correct Answer - A::D `S_(n)=-1^(2)-2^(2)+3^(2)+4^(2)-5^(2)-6^(2)+7^(2)+8^(2)-"........"+(4n-1)^(2)+(4n)^(2)` `=(3^(2)-1^(2))+(4^(2)-2^(2))+(7^(2)-5^(2))+(8^(2)-6^(2))+"........"+[{(4n-1)^(2)-(4n-3)^(2)}+{(4n)^(2)-(4n-2)^(2)}]` `=4[2+3+6+7+10+11+"......"+(4n-2)+(4n-1)]` `=8{(1+3+5+"......"+(2n-1)}+4{3+7+11+"........."+(4n-1)}` `=16n^(2)+4n=4n(4n+1),n inN` Satisfied by a and d where n=8,9, respectively. |
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