We have:
$$\int_{v_{i}}^{v} \frac{d v}{v}=-\frac{b}{m} \int_{0}^{t} d t$$
Integrating both sides, we get:
$$\ln\left(\frac{v}{v_i}\right) = -\frac{b}{m}t$$
Now, we can solve for $v$:
$$\frac{v}{v_i} = e^{-\frac{b}{m}t}$$
$$v(t) = v_i e^{-\frac{b}{m}t}$$
So,
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