Найдите сумму первых 15 членов арифметической прогрессии {An}. а5=27 а27=60

Решениеa₅ = 27a₂₇ = 60a₅ = a₁ + 4da₂₇ = a₁ + 26da₁ + 4d = 27a₁ + 26d = 6022d = 33d = 33/22d = 3/2d = 1,5a₁ + 4*1,5 = 27a₁ = 27 - 6a₁ = 21a₁₅ = a₁ + 14d = 21 + 14*1,5 = 42Sn = [(a₁ + an)*n] / 2S₁₅ = [(21 + 42)*15] / 2 S₁₅ = 472,5