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D( c )
c = 0
c = 0
c = 0
c in (-oo:0) U (0:+oo)
6/c = 20/c-5 // - 20/c-5
6/c-(20/c)+5 = 0
6/c-20*c^-1+5 = 0
-14*c^-1 = -5 // : -14
c^-1 = 5/14
-1 < 0
1/(c^1) = 5/14 // * c^1
1 = 5/14*c^1 // : 5/14
14/5 = c^1
c = 14/5
c = 14/5
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