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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
4/x = 3/15 // - 3/15
4/x-(3/15) = 0
4/x-1/5 = 0
4*x^-1 = 1/5 // : 4
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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