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D( c )
-c = 0
-c = 0
c = 0
c in (-oo:0) U (0:+oo)
12/(-c) = -4 // + 4
12/(-c)+4 = 0
-12*c^-1 = -4 // : -12
c^-1 = 1/3
-1 < 0
1/(c^1) = 1/3 // * c^1
1 = 1/3*c^1 // : 1/3
3 = c^1
c = 3
c = 3
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