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D( t )
t = 0
t = 0
t = 0
t in (-oo:0) U (0:+oo)
6/8 = 21/t // - 21/t
6/8-(21/t) = 0
6/8-21*t^-1 = 0
-21*t^-1 = -3/4 // : -21
t^-1 = 1/28
-1 < 0
1/(t^1) = 1/28 // * t^1
1 = 1/28*t^1 // : 1/28
28 = t^1
t = 28
t = 28
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