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D( x )
-x = 0
-x = 0
x = 0
x in (-oo:0) U (0:+oo)
20/(-x) = 2 // - 2
20/(-x)-2 = 0
-20*x^-1 = 2 // : -20
x^-1 = -1/10
-1 < 0
1/(x^1) = -1/10 // * x^1
1 = -1/10*x^1 // : -1/10
-10 = x^1
x = -10
x = -10
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