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InterviewSolution
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How many terms of the A.P. -6, `-(11)/2, -5ddot`are needed to give the sum -? |
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Answer» Let the required number of terms be n. Then , `a=-6,d= {(-11)/2-(-6)}=((-11)/2+6)=1/2and S_(n)= -25` `S_(n)= -25 Rightarrown/2,.[2a+ (n-1)d]=-25` `Rightarrow n/2.[ 2xx (-6)+(n-1).1/2]= -25` `Rightarrown/2.( n/2-25/2) =-25 Rightarrown/2.((n-25))/2 = -25` `Rightarrow n^(2) -25n+100=0 Rightarrow (n-5) (n-20) =0` ` Rightarrow n=5 or n=20` This shows that the sum of first five terms is -25 and that the sum of frist 20 terms is also -25. It means that the sum of all the terms from 6th to 20th is zero. |
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