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D( t )
t = 0
t = 0
t = 0
t in (-oo:0) U (0:+oo)
13/t = 91/7 // - 91/7
13/t-(91/7) = 0
13/t-13 = 0
13*t^-1 = 13 // : 13
t^-1 = 1
-1 < 0
1/(t^1) = 1 // * t^1
1 = 1*t^1 // : 1
1 = t^1
t = 1
t = 1
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