1)
а12=-28
а=-28:12
а=-2.(3)
2)
а45=-127
а=-127:45
а=-2.8(2)
а₁₂ = а₁+11d = -28; a₁ = -28-11d
а₄₅ = а₁+44d = -127; a₁ = -127-44d
-28-11d = -127-44d
-11d+44d = -127+28
33d = -99
d = -3.
а₁+11*(-3) = -28
а₁-33 = -28
а₁ = 5
