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D( t )
t = 0
t = 0
t = 0
t in (-oo:0) U (0:+oo)
20 = 60/t // - 60/t
20-(60/t) = 0
20-60*t^-1 = 0
-60*t^-1 = -20 // : -60
t^-1 = 1/3
-1 < 0
1/(t^1) = 1/3 // * t^1
1 = 1/3*t^1 // : 1/3
3 = t^1
t = 3
t = 3
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