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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
2.5 = (102-100)/x // - (102-100)/x
2.5-((102-100)/x) = 0
-2*x^-1 = -2.5 // : -2
x^-1 = 1.25
-1 < 0
1/(x^1) = 1.25 // * x^1
1 = 1.25*x^1 // : 1.25
0.8 = x^1
x = 0.8
x = 0.8
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