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D( X )
X = 0
X = 0
X = 0
X in (-oo:0) U (0:+oo)
(630/90)/X = 1 // - 1
(630/90)/X-1 = 0
7*X^-1 = 1 // : 7
X^-1 = 1/7
-1 < 0
1/(X^1) = 1/7 // * X^1
1 = 1/7*X^1 // : 1/7
7 = X^1
X = 7
X = 7
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