$\text { Đáp án: }$
$\text { a) }$ $\frac {2}{3}$ $+$ $\frac {3}{4}$ $–$ $\frac {5}{6}$
$ = $ $\frac {8}{12}$ $+$ $\frac {9}{12}$ $–$ $\frac {10}{12}$
$ = $ $\frac {7}{12}$
$\text { b) }$ $\frac {5}{3}$ $+$ $\frac {1}{3}$ $:$ $($ $\frac {–2}{3}$ $)²$
$ = $ $\frac {5}{3}$ $+$ $\frac {1}{3}$ $:$ $\frac {4}{9}$
$ = $ $\frac {5}{3}$ $+$ $\frac {1}{3}$ $×$ $\frac {9}{4}$
$ = $ $\frac {5}{3}$ $+$ $\frac {1}{4}$
$ = $ $\frac {20}{12}$ $+$ $\frac {3}{12}$
$ = $ $\frac {23}{12}$
