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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
4/5 = 12/x // - 12/x
4/5-(12/x) = 0
4/5-12*x^-1 = 0
-12*x^-1 = -4/5 // : -12
x^-1 = 1/15
-1 < 0
1/(x^1) = 1/15 // * x^1
1 = 1/15*x^1 // : 1/15
15 = x^1
x = 15
x = 15
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