InterviewSolution
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InterviewSolution
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Given a=2,d=8,sn=90, find the n and a |
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Answer» Sn = n/2 (2a + (n-1)) Sn = n/2 ( 4 + 8n -8) Sn = n/2 (8n - 4)90 = -2n + 4n^2 4n^2 - 2n - 90 = 0 2(2n^2 - n - 45 = 0)2n^2 - n - 45 = 02n^2 - 10n + 9n - 45 = 02n(n-5) 9(n-5) = 0(n-5) (2n + 9) = 0 "n" can\'t be negative or in fraction so.., n = 5 We know that Sn=n/2[2a+(n-1)d]Now,90=n/2[4+(n-1)8]90=n/2(4+8n-8)180=n(4+8n-8)180=n(8n-4)180=8n^2-4n8n^2-4n-180=04(2n^2-n-45)=02n^2-n-45=02n^2-10n+9n-45=02n(n-5)+9(n-5)=02n=-9,n=-9/2 n-5=0,n=5 |
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