CBSE Class 10 Answered
A.P
Asked by rohitkumarh94 | 15 Mar, 2010, 07:47: PM
Expert Answer
Sn = n(2a0+(n-1)d)/2
S10 = 10(2a0+9d)/2 = -80.
2a0+9d = -16 ... (1)
Sum of next 10 terms is, S20 - S10 , which is
20(2a0+19d)/2 - 10(2a0+9d)/2 = -280
2a0 + 29d = -56 ..... (2).
Solving (1) and (2),
d = - 2 and a0 = 1
Hence the AP is, 1, -1 , -3, -5, -7, ....
Regards,
Team,
TopperLearning.
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