Действительно, проверим:
a₂ = -15 + 5 = -10
a₃ = -15 + 10 = -5
a₄ = -15 + 15 = 0
a₅ = -15 + 20 = 5
a₆ = -15 + 25 = 10
a₇ = -15 + 30 = 15
a₁ + a₂ + ... + a₇ = -15 + (-10) + ... + 10 + 15 = 0
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Действительно, проверим:
aₙ = a₁ + (n - 1) · d
a₂ = -15 + 5 = -10
a₃ = -15 + 10 = -5
a₄ = -15 + 15 = 0
a₅ = -15 + 20 = 5
a₆ = -15 + 25 = 10
a₇ = -15 + 30 = 15
a₁ + a₂ + ... + a₇ = -15 + (-10) + ... + 10 + 15 = 0
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