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D( t )
t = 0
t = 0
t = 0
t in (-oo:0) U (0:+oo)
90 = 135/t // - 135/t
90-(135/t) = 0
90-135*t^-1 = 0
-135*t^-1 = -90 // : -135
t^-1 = 2/3
-1 < 0
1/(t^1) = 2/3 // * t^1
1 = 2/3*t^1 // : 2/3
3/2 = t^1
t = 3/2
t = 3/2
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