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D( m )
m = 0
m = 0
m = 0
m in (-oo:0) U (0:+oo)
80/m = 48/20 // - 48/20
80/m-(48/20) = 0
80/m-12/5 = 0
80*m^-1 = 12/5 // : 80
m^-1 = 3/100
-1 < 0
1/(m^1) = 3/100 // * m^1
1 = 3/100*m^1 // : 3/100
100/3 = m^1
m = 100/3
m = 100/3
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