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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
100 = 500/x // - 500/x
100-(500/x) = 0
100-500*x^-1 = 0
-500*x^-1 = -100 // : -500
x^-1 = 1/5
-1 < 0
1/(x^1) = 1/5 // * x^1
1 = 1/5*x^1 // : 1/5
5 = x^1
x = 5
x = 5
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