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D( t )
t = 0
t = 0
t = 0
t in (-oo:0) U (0:+oo)
66/t = 6 // - 6
66/t-6 = 0
66*t^-1 = 6 // : 66
t^-1 = 1/11
-1 < 0
1/(t^1) = 1/11 // * t^1
1 = 1/11*t^1 // : 1/11
11 = t^1
t = 11
t = 11
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