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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
400000/12 = 53000000/x // - 53000000/x
400000/12-(53000000/x) = 0
400000/12-53000000*x^-1 = 0
-53000000*x^-1 = -100000/3 // : -53000000
x^-1 = 1/1590
-1 < 0
1/(x^1) = 1/1590 // * x^1
1 = 1/1590*x^1 // : 1/1590
1590 = x^1
x = 1590
x = 1590
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