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In an AP 6th term is half the 4th term, and the 3rd term is 15. If Sn 66 then find thevalues of n. |
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Answer» 6th term=4th term/2 a+5d=(a+3d)/2 2a+10d=a+3d a+7d=0 3rd term = 15 a+2d = 15 a+7d-a-2d = 0-15 5d = -15 d = -3 a-21 = 0 a=21 Sn=66 n/2(2a+(n-1)d) = 66 n(42+(n-1)(-3)) = 132 n(42-3n+3)=132 -3n*n+45n-132 = 0 n*n-15n+44 = 0 (n-11)(n-4) = 0 n = 4 or 11 |
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