Question

Find the sum of the series 1 + 4 + 7 +......to 40terms

Answer 1

Answer:

a= 1 , d= 3 and n = 40

$s(n) = n \div 2(2a + (n - 1)d)$

$= 40 \div 2(2 \times 1 + (40 - 1)3)$

$= 20(2 + 39 \times 3)$

$= 20 \times 119$

$= 2380$

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