a,$\frac{9}{4}$-1,2+$\frac{1}{4}$
=$\frac{9}{4}$-$\frac{6}{5}$+$\frac{1}{4}$
=$\frac{10}{4}$-$\frac{6}{5}$
=$\frac{50}{20}$-$\frac{24}{20}$
=$\frac{26}{20}$
=$\frac{13}{10}$
b,$\frac{1}{3}$+1,5-$\frac{1}{4}$
=$\frac{1}{3}$+$\frac{3}{2}$-$\frac{1}{4}$
=$\frac{4}{12}$+$\frac{18}{12}$-$\frac{3}{12}$
=$\frac{19}{12}$
Chúc bạn học tốt!!!