The sum of 9 terms of an A.P is 72 and the sum of 5 terms is 20 find the sum of first n terms
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Answer:
a7+a12=97
a+4d+a+11d=97
a2+17d=97------------- ep²
subtracting ep¹from ep²
2a+17d=97
2a+12d=72
(-) (-) (-)
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5d=25
d=25/5
put d=5 in ep¹
2a+12(5)=27
2a+60=72
2a=72—60
a=72–60
a=12/2
a=6
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Answer: Sum of first n terms = n²-n
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