50. A particle moves along the x-axis in such a way that its velocity at time t > 0 is given by: v = et / t. At what value of t does v attain its minimum?






Answer is: option2

1

Solution:

v(t) = et / t

Using the quotient rule:

v'(t) = (et·t − et·1) / t² = (et(t − 1)) / t²

Set the derivative equal to zero to find critical points:

v'(t) = 0    ⟹    (et(t − 1)) / t² = 0

Since et > 0 and t² > 0 for t > 0, the equation simplifies to:
t − 1 = 0    ⟹    t = 1

For t < 1 (e.g., t = 0.5):

v'(0.5) = (e0.5(0.5 − 1)) / (0.5)² = Negative

The function is decreasing.

For t > 1 (e.g., t = 2):

v'(2) = (e2(2 − 1)) / 2² = Positive

The function is increasing.

Since the function changes from decreasing to increasing at t = 1, this point corresponds to a minimum.

The velocity v attains its minimum at t = 1.

Answer: B

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