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D( t )
t = 0
t = 0
t = 0
t in (-oo:0) U (0:+oo)
12/4 = 30/t // - 30/t
12/4-(30/t) = 0
12/4-30*t^-1 = 0
-30*t^-1 = -3 // : -30
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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