To find the sum of n terms in an arithmetic sequence, use the sum formula: \(S_n = \frac{n}{2}[2a + (n-1)d]\) or \(S_n = \frac{n}{2}(a + l)\) where l is the last term. Method 1 (using first formula): Given: a = 3, d = 7 - 3 = 4, n = 20. \(S_{20} = \frac{20}{2}[2(3) + (20-1) \times 4]\). \(S_{20} = 10[6 + 19 \times 4]\). \(S_{20} = 10[6 + 76]\). \(S_{20} = 10 \times 82 = 820\). Method 2 (using second formula): First find \(T_{20} = 3 + 19 \times 4 = 79\). Then \(S_{20} = \frac{20}{2}(3 + 79) = 10 \times 82 = 820\). Both methods give 820. In ECZ exams, you can use either formula, but the first is more versatile when you don't know the last term. The sum formula is crucial for Paper 2 problems involving total savings, total distance traveled, or cumulative quantities over time. This type of question is a staple in ECZ examinations.

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• Naomi Miselo – 31 pts

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