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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
12000/x = 10*x+800 // - 10*x+800
12000/x-(10*x)-800 = 0
12000/x-10*x-800 = 0
12000*x^-1-10*x^1-800*x^0 = 0
(12000*x^0-10*x^2-800*x^1)/(x^1) = 0 // * x^2
x^1*(12000*x^0-10*x^2-800*x^1) = 0
x^1
12000-10*x^2-800*x = 0
12000-10*x^2-800*x = 0
DELTA = (-800)^2-(-10*4*12000)
DELTA = 1120000
DELTA > 0
x = (1120000^(1/2)+800)/(-10*2) or x = (800-1120000^(1/2))/(-10*2)
x = (400*7^(1/2)+800)/(-20) or x = (800-400*7^(1/2))/(-20)
x in { (400*7^(1/2)+800)/(-20), (800-400*7^(1/2))/(-20)}
x in { (400*7^(1/2)+800)/(-20), (800-400*7^(1/2))/(-20) }
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