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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
100/x = 15 // - 15
100/x-15 = 0
100*x^-1 = 15 // : 100
x^-1 = 3/20
-1 < 0
1/(x^1) = 3/20 // * x^1
1 = 3/20*x^1 // : 3/20
20/3 = x^1
x = 20/3
x = 20/3
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