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Rashmi Bajpayee 7 years, 1 month ago
Given: Tn = 9 - 5n
T1 = a = 9 - 5 x 1 = 9 - 5 = 4
T2 = 9 - 5 x 2 = 9 - 10 = -1
Therefore, d = -1 - 4 = -5
S15 = n/2[2a + (n - 1)d]
= 15/2[2 x 4 + (15 - 1) x (-5)]
= 15/2[8 - 70]
= 15/2 x [-62]
= -465
Therefore, the sum of 15 terms of given AP is -465.
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