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D( m )
m = 0
m = 0
m = 0
m in (-oo:0) U (0:+oo)
8/m = 2/5 // - 2/5
8/m-(2/5) = 0
8/m-2/5 = 0
8*m^-1 = 2/5 // : 8
m^-1 = 1/20
-1 < 0
1/(m^1) = 1/20 // * m^1
1 = 1/20*m^1 // : 1/20
20 = m^1
m = 20
m = 20
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