alleryFind the sum of the first 20 terms of the AP 5, 9, 13, …
A900
B720
C660
D860
Answer & Solution
Correct answer: D. 860
a = 5, d = 4, n = 20. Sₙ = 20/2 [10 + 19·4] = 10 × 86 = 860.
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