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D( t )
t = 0
t = 0
t = 0
t in (-oo:0) U (0:+oo)
2.1/42 = 7/t // - 7/t
2.1/42-(7/t) = 0
2.1/42-7*t^-1 = 0
-7*t^-1 = -0.05 // : -7
t^-1 = 0.00714286
-1 < 0
1/(t^1) = 0.00714286 // * t^1
1 = 0.00714286*t^1 // : 0.00714286
139.999944 = t^1
t = 139.999944
t = 139.999944
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