А20=-21 d=-3 a19-?помогите

Question

А20=-21 d=-3 a19-?помогите

Answer 1

a₂₀ = - 21     d = - 3     a₁₉ = ?

a₂₀ = a₁₉ + d

a₁₉ = a₂₀ - d = - 21 - (- 3) = - 21 + 3 = - 18

a₁₉ = - 18