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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
40/x = 5/100 // - 5/100
40/x-(5/100) = 0
40/x-1/20 = 0
40*x^-1 = 1/20 // : 40
x^-1 = 1/800
-1 < 0
1/(x^1) = 1/800 // * x^1
1 = 1/800*x^1 // : 1/800
800 = x^1
x = 800
x = 800
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