an арифметическая прогрессия a4+a8=35, a3+a21=65 Найти a1 , d

Дано:a₄ + a₈= 35 a₃ +a₂₁ = 65 a₁-?  d - ?a₄ = a₁+3da₈ = a₁+7da₃ = a₁+2da₂₁ = a₁+20d{a₁+3d + a₁+7d = 35 {a₁+2d + a₁+20d = 65{2a₁ + 10d = 35 {2a₁ + 22d = 65{2a₁ = 35 - 10d{(35-10d) + 22d = 65(35-10d) + 22d = 6512d = 30d = 30/12d = 2,52a₁ = 35 - 10·2,52a₁ = 10a₁ = 10:2  a₁=5Ответ: а = 5; d = 2,5