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D( c )
c = 0
c = 0
c = 0
c in (-oo:0) U (0:+oo)
72/c = 18 // - 18
72/c-18 = 0
72*c^-1 = 18 // : 72
c^-1 = 1/4
-1 < 0
1/(c^1) = 1/4 // * c^1
1 = 1/4*c^1 // : 1/4
4 = c^1
c = 4
c = 4
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