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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
25.15 = 40/x // - 40/x
25.15-(40/x) = 0
25.15-40*x^-1 = 0
-40*x^-1 = -25.15 // : -40
x^-1 = 0.62875
-1 < 0
1/(x^1) = 0.62875 // * x^1
1 = 0.62875*x^1 // : 0.62875
1.59045726 = x^1
x = 1.59045726
x = 1.59045726
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