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D( t )
t = 0
t = 0
t = 0
t in (-oo:0) U (0:+oo)
20 = 200/t // - 200/t
20-(200/t) = 0
20-200*t^-1 = 0
-200*t^-1 = -20 // : -200
t^-1 = 1/10
-1 < 0
1/(t^1) = 1/10 // * t^1
1 = 1/10*t^1 // : 1/10
10 = t^1
t = 10
t = 10
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