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D( t )
t = 0
t = 0
t = 0
t in (-oo:0) U (0:+oo)
8/t = 840/210 // - 840/210
8/t-(840/210) = 0
8/t-4 = 0
8*t^-1 = 4 // : 8
t^-1 = 1/2
-1 < 0
1/(t^1) = 1/2 // * t^1
1 = 1/2*t^1 // : 1/2
2 = t^1
t = 2
t = 2
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