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D( X )
X = 0
X = 0
X = 0
X in (-oo:0) U (0:+oo)
6*(220/X) = 66 // - 66
6*(220/X)-66 = 0
1320*X^-1 = 66 // : 1320
X^-1 = 1/20
-1 < 0
1/(X^1) = 1/20 // * X^1
1 = 1/20*X^1 // : 1/20
20 = X^1
X = 20
X = 20
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