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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
(x-100)/x = 0.4 // - 0.4
(x-100)/x-0.4 = 0
(x-100)/x+(-0.4*x)/x = 0
x-0.4*x-100 = 0
0.6*x-100 = 0
(0.6*x-100)/x = 0
(0.6*x-100)/x = 0 // * x
0.6*x-100 = 0
0.6*x-100 = 0 // + 100
0.6*x = 100 // : 0.6
x = 100/0.6
x = 166.66666667
x = 166.66666667
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