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Арифметическая прогрессия

a2+a6=26 => 2a1+6d=26 => a1+3d=13 => a4 = 13a2=a1+d=9a4=a1+3d=13Значит, 2d=4 => d = 2a1=a2-d=9-2=7S6 = (a1+a6)*6/2 = 3 (7+17) = 3 * 24 = 72a7 = a1+6d = 7+6*2=7+12=19Ответ: 19, 13, 2, 723) a5+a9=6 => 2a1+12d=6 => a1+6d=3 => a7 = 3a1+6d=3a1+4d=92d=-6 = > d = -3a1+4d=9 => a1=9-4d = 9+12=21a4=a1+3d=21+3*(-3) = 21-9=12a8 = a9 - d = -3 - (-3) = 0S8 = (a1+a8) * 8 / 2 = a1 * 4 = 21 * 4 = 84Ответ: 3, 12, -3, 84