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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
40/x = 1/2 // - 1/2
40/x-(1/2) = 0
40/x-1/2 = 0
40*x^-1 = 1/2 // : 40
x^-1 = 1/80
-1 < 0
1/(x^1) = 1/80 // * x^1
1 = 1/80*x^1 // : 1/80
80 = x^1
x = 80
x = 80
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