Odpowiedź:
zad 3
a₁ = x+ 8
a₂= 2x+ 3
a₃= 4x- 5
a₃ - a₂ = a₂ - a₁
4x- 5 - 2x - 3 = 2x + 3 - x - 8
2x- 8 = x - 5
2x - x = - 5 + 8
x = 3
a₁ = x + 8 = 3 + 8 = 11
a₂ = 2x+ 3 = 2 * 3 + 3 = 6 + 3 = 9
a₃ = 4x- 5= 4 * 3 - 5 = 12 - 5 = 7
r - różnica ciągu = a₂ - a₁ = 9 - 11 = - 2
an = a₁ + (n - 1) * r = 11 + (n - 1) * ( - 2) = 11 - 2n + 2 = - 2n + 13
a₈ = a₁ + 7r = 11 + 7 * (- 2)= 11 - 14 = - 3
S₈ = (a₁ + a₈) * 8/2 = (11 - 3) * 4 = 8 * 4 = 32
