Question

In an A.P. the sum of first ten terms is –80 and the sum of next ten terms is –280. Find the A.P

Answer 1

let a is the first term and d is the common difference of AP
now use formula,
Sn=n/2{2a+(n-1) d}

now
S10=10/2 {2a+(10-1) d}
-80=5 (2a+9d)
2a+9d =-16 ---------------(1)

again
S20-S10=-280

20/2 {2a+19d} +80=-280

10 (2a+19d)=-360

2a+19d=-36 -----------------(2)

solve both equation
d=-2
a=1

hence 1, -1,-3,-5 ........

Answer 2

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