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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
6/15 = 8/x // - 8/x
6/15-(8/x) = 0
6/15-8*x^-1 = 0
-8*x^-1 = -2/5 // : -8
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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