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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
((0.4-x)*(0.4-x)^2)/x = 0
((0.4-x)^3)/x = 0
0.4-x = 0 // - 0.4
-x = -0.4 // * -1
x = 0.4
x in { 0.4, 0.4, 0.4 }
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