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D( t )
t = 0
t = 0
t = 0
t in (-oo:0) U (0:+oo)
4/t = 5/7 // - 5/7
4/t-(5/7) = 0
4/t-5/7 = 0
4*t^-1 = 5/7 // : 4
t^-1 = 5/28
-1 < 0
1/(t^1) = 5/28 // * t^1
1 = 5/28*t^1 // : 5/28
28/5 = t^1
t = 28/5
t = 28/5
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