Find the sum of the first 20 terms of the Arithmetic Progression: 3, 7, 11, 15, ...
Answer and explanation
Correct answer: D. 820
- Step 1: Identify a = 3, d = 4, n = 20. Step 2. Sum formula: Sn = 0.5 n [2a + (n - 1)d]. Step 3. Substitute values: S20 = 0.5(20)[2(3) + (19)(4)] = 10[6 + 76] = 10(82) = 820. Answer: 820. Why not the others: 860 uses d = 5; 780 uses n = 19; 900 is an arithmetic error.
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