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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
0.9 = (x-4)/x // - (x-4)/x
0.9-((x-4)/x) = 0
(-1*(x-4))/x+0.9 = 0
(-1*(x-4))/x+(0.9*x)/x = 0
0.9*x-1*(x-4) = 0
4-0.1*x = 0
(4-0.1*x)/x = 0
(4-0.1*x)/x = 0 // * x
4-0.1*x = 0
4-0.1*x = 0 // - 4
-0.1*x = -4 // : -0.1
x = -4/(-0.1)
x = 40
x = 40
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