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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
30/15 = 8/x // - 8/x
30/15-(8/x) = 0
30/15-8*x^-1 = 0
-8*x^-1 = -2 // : -8
x^-1 = 1/4
-1 < 0
1/(x^1) = 1/4 // * x^1
1 = 1/4*x^1 // : 1/4
4 = x^1
x = 4
x = 4
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