Find the sum of first 39 terms of Arithmetic sequence 5n-3
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Answer:
a1 = 5(1)-3 = 2
a2= 5(2)-3 = 7
a3= 5(3)-3 = 12
d = 7-2= 5
Sn = n/2 [2a + (n-1)d]
S39 = 39/2 [2×2 + (39-1)5]
= 39/2 [4 + 38×5]
= 39/2 (4+190)
= 39/2 × 194
= 39×97
= 3783
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