Дана арифметическая прогрессия an найдите d, если s9=81, а1=-1

S9 = (2a1 + 8d) / 2 * 9 = 9(a1 + 4d) 9(4d - 1) = 81 4d - 1 = 9 4d = 10 d = 2.5

D=an+1-anпо формуле найдешь d

Одз {x-1> 0⇒x> 1 {x²-8x+6≥0  d=64-24=40 x1=4-√10 u xx2=4+√10⇒x≤4-√10 u x≥4+√10{x²-4x+3≤0  x1+x2=4 u x1*x2=3⇒x1=1 u x2=3⇒1≤x≤ 3 \\\\\\\\\\\\\\\\\\                     \\\\\\\\\\\\           \\\\\\\\\\\\\\\\\\\\\\\\\\\\\                                         \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ -√+√

Одз {x-1> 0⇒x> 1 {x²-8x+6≥0  d=64-24=40 x1=4-√10 u xx2=4+√10⇒x≤4-√10 u x≥4+√10 {x²-4x+3≤0  x1+x2=4 u x1*x2=3⇒x1=1 u x2=3⇒1≤x≤ 3 \\\\\\\\\\\\\\\\\\                     \\\\\\\\\\\\           \\\\\\\\\\\\\\\\\\\\\\\\\\\\\ -√+√                                         \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ общего решения нет