20. Using the general property d dx exp( bx )= b exp( bx ), we write v = dx dt = µ d (19 t ) dt ¶ · e − t +(19 t ) · µ de − t dt ¶ . If a concern develops about the appearance of an argument of the exponential ( − t ) apparently having units, then an explicit factor of 1 /T where T = 1 second can be inserted and carried through the computation (which does not change our answer). The result of this diFerentiation is
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