Q3 of 85 Page 104

For a given A.P. with

S20 = 100, d = —2, find a.

Formula used.


Sn = [2a + (n–1)d]


⇒ S20 = 100


⇒ n = 20


⇒ d = –2


S20 = [2a + (20–1)(–2)]


100 = 10[2a–38]


100 = 20[a–19]


= a–19


5 = a–19


a = 19 + 5


a = 24


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