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D( t )
t = 0
t = 0
t = 0
t in (-oo:0) U (0:+oo)
27/7 = 81/t // - 81/t
27/7-(81/t) = 0
27/7-81*t^-1 = 0
-81*t^-1 = -27/7 // : -81
t^-1 = 1/21
-1 < 0
1/(t^1) = 1/21 // * t^1
1 = 1/21*t^1 // : 1/21
21 = t^1
t = 21
t = 21
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