Question

In an AP the sum of the first ten terms is -80 and the sim of its next ten terms is -280 . Find the AP ?
Please fast it's urgent

Answer 1

2a+9d=-16
2a+29d=-56
20d=-40
d=-2
a=1

Answer 2

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......