Answers

2014-11-25T05:38:50+05:30
Sn = n/2 [2a + (n-1)d]

Given a = 1 , d = 1/3 - 1 = -2/3 , n = 15

S₁₅ = 15/2( 2 - 28/3 )

      = 15/2 × -22/3  
      = -55
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