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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
3/40 = 12/x // - 12/x
3/40-(12/x) = 0
3/40-12*x^-1 = 0
-12*x^-1 = -3/40 // : -12
x^-1 = 1/160
-1 < 0
1/(x^1) = 1/160 // * x^1
1 = 1/160*x^1 // : 1/160
160 = x^1
x = 160
x = 160
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