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9.If the sum of first 15 terms of an A.Ris 675 and its first term is 10,then find 25th term. |
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Answer» a=10,s15=675,Sn=n/2(2a+(n-1)d)675=15/2(2×10+(15-1) d) 675 ×2/15=(20+14d)45×2=20+14d90-20=14d70=14d d=70/14d=5an=a +(n-1) dan=10+(25-1) 5an=10+24×5an=10+120an=130 |
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