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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
15/x = 7/13 // - 7/13
15/x-(7/13) = 0
15/x-7/13 = 0
15*x^-1 = 7/13 // : 15
x^-1 = 7/195
-1 < 0
1/(x^1) = 7/195 // * x^1
1 = 7/195*x^1 // : 7/195
195/7 = x^1
x = 195/7
x = 195/7
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