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Un= a + (n-1)b
395=205+5(n-1)
190= 5n -5
195 = 5n
39 = n
S39 = n/2 x (a + Un)
= 39/2 x (205 + 395)
= 39/2 x (600)
= 39 x 600
2
= 39 x 300
= 11700