Q. 234.0( 5 Votes )

In an A.P. the su

Answer :

Let the first term of AP be ‘a’ and common difference be ‘d’.


We know, sum of ‘n’ terms of an AP is



Given,


Sum of first test ten terms, S10 = 80 ……[1]



5(2a + 9d) = 80


2a + 9d = 16 ……[2]


Also, sum of next ten terms = 280


S20 – S10 = 280


S20 – 80 = 280 [From 1]


S20 = 360



2a + 19d = 36 ……[3]


On subtracting [2] from [3]


2a + 19d – 2a – 9d = 36 – 16


10d = 20


d = 2


Putting d = 2 in [2]


2a + 18 = 16


a = –1


Hence, AP is


–1, –1 + 2, –1 + 2(2), …


–1, 1, 3, …


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