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D( X )
X = 0
X = 0
X = 0
X in (-oo:0) U (0:+oo)
100*(8/X) = 80 // - 80
100*(8/X)-80 = 0
800*X^-1 = 80 // : 800
X^-1 = 1/10
-1 < 0
1/(X^1) = 1/10 // * X^1
1 = 1/10*X^1 // : 1/10
10 = X^1
X = 10
X = 10
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