# How do you use the second fundamental theorem of Calculus to find the derivative of given int cos(t) / t dt  from [3, x]?

$= \frac{\cos x}{x}$
The 2nd FTC states that $\frac{d}{\mathrm{dx}} {\int}_{a}^{x} f \left(t\right) \mathrm{dt} = f \left(x\right)$
So here $\frac{d}{\mathrm{dx}} {\int}_{3}^{x} \cos \frac{t}{t} \mathrm{dt} = \frac{\cos x}{x}$