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b = 3
Un = 399
201,204,...,399
Un = a +(n-1)b
399 = 201 + (n-1)3
399-201 = 3n-3
198+3 = 3n
201 = 3n
n = 67
Sn = n/2(a + Un)
Sn = 67/2 (201+399)
Sn = 67/2 x 600
Sn = 20.100