Odpowiedź:
a₂ = a₁ + r = 2
a₃ = a₁ + 2r = 2/3
układ równań
a₁ + r = 2
a₁ + 2r = 2/3
odejmujemy równania
a₁ - a₁ + r - 2r = 2 - 2/3
- r = 1 1/3
r = - 1 1/3
a₁ + r = 2
a₁ - 1 1/3 = 2
a₁ = 2 + 1 1/3 = 3 1/3
a₅ = a₁ + 4r = 3 1/3 + 4 * (- 1 1/3) = 3 1/3 - 4 * 4/3 = 3 1/3 - 16/3 =
= 3 1/3 - 5 1/3 = - 2
S₅ = (a₁ + a₅) * 5/2 = ( 3 1/3 - 2 ) * 2 1/2 = 1 1/3 * 5/2 = 4/3 * 5/2 =
= 20/6 = 3 2/6 = 3 1/3
