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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
-80/x = -3 // + 3
3-80/x = 0
-80*x^-1 = -3 // : -80
x^-1 = 3/80
-1 < 0
1/(x^1) = 3/80 // * x^1
1 = 3/80*x^1 // : 3/80
80/3 = x^1
x = 80/3
x = 80/3
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