а₁ = 1 по условию
а₄ = а₁+3d = 1+3d = –2
1+3d=-2; 3d=-2-1; 3d=-3; d=–1
a₂₀ = a₁+19d = 1+19*(-1) = 1-19 = –18
S₂₀ = (a₁+a₂₀)/2*20 = (1-18)*10 = –170
ответ: –170
а₁ = 1 по условию
а₄ = а₁+3d = 1+3d = –2
1+3d=-2; 3d=-2-1; 3d=-3; d=–1
a₂₀ = a₁+19d = 1+19*(-1) = 1-19 = –18
S₂₀ = (a₁+a₂₀)/2*20 = (1-18)*10 = –170
ответ: –170