Question
Mon March 15, 2010 By: Rohit Kumar

A.P

Expert Reply
Tue March 16, 2010

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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