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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
5/7 = 15/x // - 15/x
5/7-(15/x) = 0
5/7-15*x^-1 = 0
-15*x^-1 = -5/7 // : -15
x^-1 = 1/21
-1 < 0
1/(x^1) = 1/21 // * x^1
1 = 1/21*x^1 // : 1/21
21 = x^1
x = 21
x = 21
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