Find the sum of a = (a-b)/(a+b) d = (3a-2b)/(a+b) - (a-b)/(a+b)

Find the sum
Solution :
a = (a-b)/(a+b)
d = (3a-2b)/(a+b) - (a-b)/(a+b)
  d = [3a - 2b -(a - b)]/(a + b) 
  d = [3a - 2b -a + b]/(a + b) 
  d  = (2a - b) / (a + b)
Sn  = (n/2)[2a + (n - 1)d]  

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