The sum of first 10 terms of an AP is -150 and the sum of its next 10 terms is -550. Find the AP.
[CBSE 2010]
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Here is your answer:
Let the First term of A.P=a
And common difference =d
S
30
=
2
30
[2a+(30−1)d]
S
20
=
2
20
[2a(20−1)d]
S
10
=
2
10
[2a+(10−1)d]
R.H.S=3[S
20
−S
10
]
=3[[
2
20
(2a+20−1)d]−[
2
10
(2a+(10−1)d]]
=3[20a+190d−10a−45d]
=15(2a+29d)
=
2
30
[2a+(30−1)d]
=S
30
=L.H.S
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