Let’s use the formula for the sum of the first n terms of an arithmetic progression.
Sn = ((a1 + an) / 2) * n, where a1 is the first term, an is the n term of the sequence (we have this 15th term)
Find the 15th term of the sequence by the formula an = a1 + d (n – 1).
a15 = 4 + 3 * (15 – 1) = 4 + 3 * 14 = 4 + 42 = 46.
S15 = ((4 + 46) / 2) * 15 = (50/2) * 15 = 25 * 15 = 375.
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