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