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D( t )
t = 0
t = 0
t = 0
t in (-oo:0) U (0:+oo)
690/t = 30 // - 30
690/t-30 = 0
690*t^-1 = 30 // : 690
t^-1 = 1/23
-1 < 0
1/(t^1) = 1/23 // * t^1
1 = 1/23*t^1 // : 1/23
23 = t^1
t = 23
t = 23
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