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260lal AP whose n" term is given by an 3n -216. Ife sum of first 7 terms of an AP is 49 and that of 17 terms is 289, find the sum of first n terms.Prove that: |
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Answer» sum of 7 terms=7/2(2a+6d)=497(a+3d)=49so a+3d=7sum of 17 terms=17/2(2a+16d)=28917(a+8d)=289so a+8d=17a+8d-a-3d=17-7=105d=10so d=2a+6=7 so a=1sum of n terms=n/2(2+(n-1)2)=n/2(2+2n-2) =n^2 |
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