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D( c )
c = 0
c = 0
c = 0
c in (-oo:0) U (0:+oo)
13/c = 39/60 // - 39/60
13/c-(39/60) = 0
13/c-13/20 = 0
13*c^-1 = 13/20 // : 13
c^-1 = 1/20
-1 < 0
1/(c^1) = 1/20 // * c^1
1 = 1/20*c^1 // : 1/20
20 = c^1
c = 20
c = 20
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