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D( c )
c = 0
c = 0
c = 0
c in (-oo:0) U (0:+oo)
2.1/1.6 = 6.3/c // - 6.3/c
2.1/1.6-(6.3/c) = 0
2.1/1.6-6.3*c^-1 = 0
-6.3*c^-1 = -1.3125 // : -6.3
c^-1 = 0.20833333
-1 < 0
1/(c^1) = 0.20833333 // * c^1
1 = 0.20833333*c^1 // : 0.20833333
4.80000008 = c^1
c = 4.80000008
c = 4.80000008
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