Question

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

Answer 1

Step-by-step explanation:

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let the first term and d is the common differences of AP

= 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 this two equation

then..

we found

d=-2

a=1

so...

AP=1,-1,-3,-5......

Answer 2

hope it helps............