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D( x )
x-65 = 0
x-65 = 0
x-65 = 0
x-65 = 0 // + 65
x = 65
x in (-oo:65) U (65:+oo)
400 = 8000/(x-65) // - 8000/(x-65)
400-(8000/(x-65)) = 0
400-8000*(x-65)^-1 = 0
400-8000/(x-65) = 0
(400*(x-65))/(x-65)-8000/(x-65) = 0
400*(x-65)-8000 = 0
400*x-34000 = 0
(400*x-34000)/(x-65) = 0
(400*x-34000)/(x-65) = 0 // * x-65
400*x-34000 = 0
400*x-34000 = 0 // + 34000
400*x = 34000 // : 400
x = 34000/400
x = 85
x = 85
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