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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
6/12 = 40/x // - 40/x
6/12-(40/x) = 0
6/12-40*x^-1 = 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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