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D( h )
h = 0
h = 0
h = 0
h in (-oo:0) U (0:+oo)
3.5/h = -0.2 // + 0.2
3.5/h+0.2 = 0
3.5*h^-1 = -0.2 // : 3.5
h^-1 = -0.05714286
-1 < 0
1/(h^1) = -0.05714286 // * h^1
1 = -0.05714286*h^1 // : -0.05714286
-17.49999913 = h^1
h = -17.49999913
h = -17.49999913
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