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D( t )
t = 0
t = 0
t = 0
t in (-oo:0) U (0:+oo)
7/t = 23/69 // - 23/69
7/t-(23/69) = 0
7/t-1/3 = 0
7*t^-1 = 1/3 // : 7
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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