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D( X )
X = 0
X = 0
X = 0
X in (-oo:0) U (0:+oo)
1-(6.65/X) = 0.25 // - 0.25
1-(6.65/X)-0.25 = 0
1-6.65*X^-1-0.25 = 0
-6.65*X^-1 = -0.75 // : -6.65
X^-1 = 0.11278195
-1 < 0
1/(X^1) = 0.11278195 // * X^1
1 = 0.11278195*X^1 // : 0.11278195
8.86666705 = X^1
X = 8.86666705
X = 8.86666705
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