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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
6/8 = 15/x // - 15/x
6/8-(15/x) = 0
6/8-15*x^-1 = 0
-15*x^-1 = -3/4 // : -15
x^-1 = 1/20
-1 < 0
1/(x^1) = 1/20 // * x^1
1 = 1/20*x^1 // : 1/20
20 = x^1
x = 20
x = 20
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