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a₇ = a₁ + 6r
a₁ + 2r = 15
a₁ + 6r = 8
odejmujemy stronami
a₁ - a₁ + 2r - 6r = 15 - 8
- 4r = 7
r = - 7/4
a₃ = a₁ + 2r
15 = a₁ + 2 * - 7/4 = a₁ - 14/4
a₁ = 15 + 14/4 = 15 + 3 1/2 = 18 1/2
sprawdzenie
a₇ = a₁ + 6r = 18 1/2 + 6 * - 7/4 = 18 1/2 - 42/4 = 18 1/2 - 10 1/2 = 8