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D( t )
t = 0
t = 0
t = 0
t in (-oo:0) U (0:+oo)
42/72 = 7/t // - 7/t
42/72-(7/t) = 0
42/72-7*t^-1 = 0
-7*t^-1 = -7/12 // : -7
t^-1 = 1/12
-1 < 0
1/(t^1) = 1/12 // * t^1
1 = 1/12*t^1 // : 1/12
12 = t^1
t = 12
t = 12
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