a: ĐKXĐ: tan(3x-1)<>0
=>\(\left\{{}\begin{matrix}3x-1< >\dfrac{\Pi}{2}+k\Pi\\3x-1< >k\Pi\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x< >\dfrac{1}{3}\left(\dfrac{\Pi}{2}+k\Pi+1\right)\\x< >\dfrac{1}{3}\left(k\Pi+1\right)\end{matrix}\right.\)
b: ĐKXĐ: (1-sinx)/(1+cosx)>=0
=>1-sinx>=0
=>sinx<=1(luôn đúng)