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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
0.3/1 = 12/x // - 12/x
0.3/1-(12/x) = 0
0.3-12*x^-1 = 0
-12*x^-1 = -0.3 // : -12
x^-1 = 0.025
-1 < 0
1/(x^1) = 0.025 // * x^1
1 = 0.025*x^1 // : 0.025
40 = x^1
x = 40
x = 40
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