find the sum of the series 103+101+99+....49
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d = - 2
L = 49
=> a +(n-1)d = 49
=> 103 + (n-1) (-2) = 49
=> (n-1)(-2) = - 54
=> n-1 = 27
=> n = 28
Sn = n / 2[ a + L]
S28 = 28/2 [ 103 + 49]
= 14 * 152
= 2128