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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
2/5 = 100/x // - 100/x
2/5-(100/x) = 0
2/5-100*x^-1 = 0
-100*x^-1 = -2/5 // : -100
x^-1 = 1/250
-1 < 0
1/(x^1) = 1/250 // * x^1
1 = 1/250*x^1 // : 1/250
250 = x^1
x = 250
x = 250
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