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D( x )
0.3-x = 0
0.3-x = 0
0.3-x = 0
0.3-x = 0 // - 0.3
-x = -0.3 // * -1
x = 0.3
x in (-oo:0.3) U (0.3:+oo)
(x/(0.3-x))*(0.4-x) = 0
(x*(0.4-x))/(0.3-x) = 0
( 0.4-x )
0.4-x = 0 // - 0.4
-x = -0.4 // * -1
x = 0.4
( x )
x = 0
x in { 0.4, 0 }
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