A₁ + a₂ + a₃ = 15
a₁ · a₂ · a₃ = 80
a₁ + a₁ + d + a₁ + 2d = 15
a₁· (a₁ + d) · (a₁ + 2d) = 80
3a₁ + 3d = 15
a₁· (a₁ + d) · (a₁ + 2d) = 80
a₁ + d = 5
a₁· (a₁ + d) · (a₁ + 2d) = 80
d = 5 - a₁
a₁· (a₁ + 5 - a₁) · (a₁ + 2(5 - a₁)) = 80
a₁ · 5 · (10 - a₁) = 80
50a₁ - 5a₁² - 80 = 0
a₁² - 10a₁ + 16 = 0
D = 100 - 64 = 36
a₁ = (10 - 6)/2 = 2 или a₁ = (10 + 6)/2 = 8
d = 3 d = - 3