Odpowiedź:
2.41
a)
a₇ + a₁₃ = 14
a₁₀ + a₂₂ = 10
a₁ + 6r + a₁ + 12r = 14
a₁ + 9r + a₁ + 21r = 10
2a₁ + 18r = 14
2a₁ + 30r = 10
Odejmujemy równania
2a₁ - 2a₁ + 18r - 30r = 14 - 10
- 12r = 4
12r = - 4
r = - 4/12 = - 1/3
2a₁ + 18r = 14
2a₁ + 18 * (- 1/3) = 14
2a₁ - 6 = 14
2a₁ = 14 + 6 = 20
a₁ = 20 : 2 = 10
Odp: a₁ = 10 , r = - 1/3
