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D( t )
t = 0
t = 0
t = 0
t in (-oo:0) U (0:+oo)
20/t = 4 // - 4
20/t-4 = 0
20*t^-1 = 4 // : 20
t^-1 = 1/5
-1 < 0
1/(t^1) = 1/5 // * t^1
1 = 1/5*t^1 // : 1/5
5 = t^1
t = 5
t = 5
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