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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
0.8 = 40000/x // - 40000/x
0.8-(40000/x) = 0
0.8-40000*x^-1 = 0
-40000*x^-1 = -0.8 // : -40000
x^-1 = 0.00002
-1 < 0
1/(x^1) = 0.00002 // * x^1
1 = 0.00002*x^1 // : 0.00002
50000 = x^1
x = 50000
x = 50000
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