А1+а6=20, а2+а3=17; a6=a1+5d, a2=a1+d, a3=a1+2d => 2a1+5d=20, 2a1+3d=17 Вычитаем из 1го уравнения 2ое: 2a1+5d-2a1-3d=20-17 => 2d=3 => d=1,5, 2a1+5d=20, 2a1+5*1,5=20 => a1 = (20-7,5)/2 =6,25
(2/9-2.6)*4.5=2/9*9/2-2.6*4.5=1-11.7=-10.7
D=a2-a1=a3-a2=a4-a3=.......
d=9-3=15-9=--------
d=6
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3у²+у=7
3у²+у-7 =0
D= 1² - 4*3*(-7) = 1+84=85
x₁= (-1-√85) /(2*3) = (-1-√85)/6
x₂=(-1+√85)/6