Question

Find the sum of first 39 terms of Arithmetic sequence 5n-3

Answer 1

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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