Question

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

Answer 1

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

Answer 2

Answer: Sum of first n terms = n²-n

Explanation: