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\begin{aligned}&\begin{array}{l}\frac{\text { d }}{\text { dt }}\left(\frac{3 t-2}{5 t+1}\right)=\frac{(5 t+1) D(3 t-2)-(3 t-2) D(5 t+1)}{(5 t+1)^{2}}=\frac{(5 t+1)(3)-(3 t-2)(5)}{(5 t+1)^{2}}=\frac{13}{(5 t+1)^{2}} \\ D\left(\frac{\cos (x)}{x}\right)=\frac{x D(\cos (x))-\cos (x) D(x)}{(x)^{2}}=\frac{x(-\sin (x))-\cos (x)(1)}{x^{2}}=\frac{-x \cdot \sin (x)-\cos (x)}{x^{2}} \end{array} \end{aligned}