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D( t )
t = 0
t = 0
t = 0
t in (-oo:0) U (0:+oo)
-80/t = 8 // - 8
-80/t-8 = 0
-80*t^-1 = 8 // : -80
t^-1 = -1/10
-1 < 0
1/(t^1) = -1/10 // * t^1
1 = -1/10*t^1 // : -1/10
-10 = t^1
t = -10
t = -10
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