Find the 20th term of 3, 7, 11, 15, …
Use the nth-term formula for an arithmetic sequence with first term 3 and common difference 4.
Solution
- Given a₁ = 3, common difference d = 4, n = 20
- nth term aₙ = a₁ + (n−1)d = 3 + 19·4 = 79
- Sum of n terms Sₙ = n/2·(a₁ + aₙ) = 20/2·(3 + 79) = 820
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