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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
12/5 = 36/x // - 36/x
12/5-(36/x) = 0
12/5-36*x^-1 = 0
-36*x^-1 = -12/5 // : -36
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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