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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
3/10 = 12/x // - 12/x
3/10-(12/x) = 0
3/10-12*x^-1 = 0
-12*x^-1 = -3/10 // : -12
x^-1 = 1/40
-1 < 0
1/(x^1) = 1/40 // * x^1
1 = 1/40*x^1 // : 1/40
40 = x^1
x = 40
x = 40
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