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