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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
2400/x = 300-(6*x) // - 300-(6*x)
6*x+2400/x-300 = 0
6*x^1+2400*x^-1-300*x^0 = 0
(6*x^2-300*x^1+2400*x^0)/(x^1) = 0 // * x^2
x^1*(6*x^2-300*x^1+2400*x^0) = 0
x^1
6*x^2-300*x+2400 = 0
6*x^2-300*x+2400 = 0
DELTA = (-300)^2-(4*6*2400)
DELTA = 32400
DELTA > 0
x = (32400^(1/2)+300)/(2*6) or x = (300-32400^(1/2))/(2*6)
x = 40 or x = 10
x in { 10, 40}
x in { 10, 40 }
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