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D( h )
h = 0
h = 0
h = 0
h in (-oo:0) U (0:+oo)
8/5 = 40/h // - 40/h
8/5-(40/h) = 0
8/5-40*h^-1 = 0
-40*h^-1 = -8/5 // : -40
h^-1 = 1/25
-1 < 0
1/(h^1) = 1/25 // * h^1
1 = 1/25*h^1 // : 1/25
25 = h^1
h = 25
h = 25
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