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D( t )
t = 0
t = 0
t = 0
t in (-oo:0) U (0:+oo)
160/t = 20 // - 20
160/t-20 = 0
160*t^-1 = 20 // : 160
t^-1 = 1/8
-1 < 0
1/(t^1) = 1/8 // * t^1
1 = 1/8*t^1 // : 1/8
8 = t^1
t = 8
t = 8
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