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D( x )
75-x = 0
75-x = 0
75-x = 0
75-x = 0 // - 75
-x = -75 // * -1
x = 75
x in (-oo:75) U (75:+oo)
192/48 = x/(75-x) // - x/(75-x)
192/48-(x/(75-x)) = 0
(-1*x)/(75-x)+4 = 0
(-1*x)/(75-x)+(4*(75-x))/(75-x) = 0
4*(75-x)-1*x = 0
300-5*x = 0
(300-5*x)/(75-x) = 0
(300-5*x)/(75-x) = 0 // * 75-x
300-5*x = 0
300-5*x = 0 // - 300
-5*x = -300 // : -5
x = -300/(-5)
x = 60
x = 60
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