The Sum of first seven terms of an AP is 49 and the sum of first seventeen terms is 289 find the sum of first 20 terms​

S7=49

a+6d=49

a=49-6d1

S17=289

a+16d=289

substituting value of a

49-6d+16d=289

10d=289-49

d=240/10=24

:. d=24

substituting value of d in equation 1

a=49-6×24=-95

now

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