a)
\sqrt{\frac{5}{x-3}}
\frac{5}{x-3}>0
x-3>0
x>3
D: x\in(3;+\infty)
b)
\sqrt{\frac{x+2}{4x+1}}
\frac{x+2}{4x+1}\geq0
x+2\geq0 i 4x+1>0
x\geq-2 i 4x>-1
x\geq-2 i x>-\frac{1}{4}
lub
x+2\leq0 i 4x+1<0
x\leq-2 i 4x<-1
x\leq-2 i x<-\frac{1}{4}
D: x\in(-\infty;-2\rangle\cup \(\frac{1}{4};+\infty )