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Review: Sum of a sequence = average × number of terms
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Learn it Sum of an arithmetic sequence = average × number of terms. The average is (first + last) ÷ 2. Count the terms with (last − first) ÷ d + 1.
Example: Find $5 + 9 + 13 + \dots + 97$.
- Number of terms: $(97 - 5) \div 4 + 1 = 24$.
- Average: $(5 + 97) \div 2 = 51$.
- Sum $= 51 \times 24 = 1{,}224$.
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