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D( c )
c = 0
c = 0
c = 0
c in (-oo:0) U (0:+oo)
4/c-(5/7) = 0
4/c-5/7 = 0
4*c^-1 = 5/7 // : 4
c^-1 = 5/28
-1 < 0
1/(c^1) = 5/28 // * c^1
1 = 5/28*c^1 // : 5/28
28/5 = c^1
c = 28/5
c = 28/5
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