$a)\frac{\cos x\left(\frac{\sin x}{\cos x}-1\right)}{\cos x\left(\frac{\sin x}{\cos x}+1\right)}=\frac{\frac{\sin x}{\cos x}-1}{\frac{\sin x}{\cos x}+1}=\frac{\mathrm{tg}x-1}{\mathrm{tg}x+1}=$  

$\mathrm{tg}x=4$  

$=\frac{4-1}{4+1}=\frac{3}{5}$