Решение
<span>a</span>₂ <span>= a</span>₁ <span>+ d
</span><span>a₉ </span><span>=a₁ </span><span>+ </span><span>8d
</span><span>a</span>₂ <span>= 3a</span>₉
<span>3(a</span>₁ <span>+ 8d) = a</span>₁ <span>+ d
</span><span>3a</span>₁ <span>+ 24d = a</span>₁ <span>+ d
</span><span>2a</span>₁ <span>+ 23d = 0
</span><span>2a</span>₁ = - 23d
<span>a</span>₁ = - 11,5d
<span>Sn = [2a</span>₁ + d*(n - 1)*n]/2
S₂₀ = [(a₁ + a₁ + 19d)*20]/2 = (a₁ + a₂₀)*10
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F'(x)=6x^2+5x-1 формула (x^n)'=nx^(n-1)
f'(x)>0
6x^2+5x-1>0 6x^2+5x-1=0 a+c=b⇒x1=-1 x2=-c/a=1/6
(x+1)(x-1/6)>0
+ -1 - 1/6 +
ответ (-00,-1)∪(1/6,+00)