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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
40 = (23.1-x)/x // - (23.1-x)/x
40-((23.1-x)/x) = 0
(-1*(23.1-x))/x+40 = 0
(-1*(23.1-x))/x+(40*x)/x = 0
40*x-1*(23.1-x) = 0
41*x-23.1 = 0
(41*x-23.1)/x = 0
(41*x-23.1)/x = 0 // * x
41*x-23.1 = 0
41*x-23.1 = 0 // + 23.1
41*x = 23.1 // : 41
x = 23.1/41
x = 0.56341463
x = 0.56341463
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