a_n = 5n — 1
a₁ = 5 — 1 = 4
a₂ = 10 — 1 = 9
a₃ = 15 — 1 = 14
a₄ = 20 — 1 = 19

b_n = n/n+3
b₁ = 1/1+3 = 1/4
b₂ = 2/2+3 = 2/5
b₃ = 3/3+3 = 3/6
b₄ = 4/4+3 = 4/7

c_n = n² — 1
c₁ = 1 — 1 = 0
c₂ = 4 — 1 = 3
c₃ = 9 — 1 = 8
c₄ = 16 — 1 = 15

x_n = 3n — 1
x₁ = 1
x₂ = 3
x₃ = 9
x₄ = 27

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a₁ = 4
d = ?
a₅ = ?

a₅ = a₁ + 4d = 4 + 4 * 3 = 16

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a₁ = 1,3
d = -0,2
a₅ = ?

a₅ = a₁ + 4d = 1,3 + 4 * (-0,2) = 0,5