Đáp án:
a) $\dfrac{2}{3}$ $+$ $\dfrac{5}{2}$ $-$ $\dfrac{3}{4}$
$=$ $\dfrac{2×4}{3×4}$ $+$$\dfrac{5 ×6}{2 × 6}$ $-$ $\dfrac{3×3}{4×3}$
$=$ $\dfrac{8}{12}$ $+$ $\dfrac{30}{12}$ $-$ $\dfrac{9}{12}$
$=$ $\dfrac{8 + 30 - 9}{12}$
$=$ $\dfrac{29}{12}$
b) $[$$\dfrac{6}{7}$ $-$ $\dfrac{4}{7}$$]$ $÷$ $\dfrac{2}{5}$
= $\dfrac{2}{7}$ $÷$ $\dfrac{2}{5}$
= $\dfrac{2}{7}$ $×$ $\dfrac{5}{2}$
= $\dfrac{5}{7}$
c) $\dfrac{4}{5}$ $-$ $\dfrac{1}{2}$ $+$ $\dfrac{1}{3}$
= $\dfrac{4×6}{5×6}$ $-$ $\dfrac{1×15}{2×15}$ $+$ $\dfrac{1×10}{3×10}$
= $\dfrac{24}{30}$ $-$ $\dfrac{15}{30}$ $+$ $\dfrac{10}{30}$
= $\dfrac{24-15+10}{30}$
= $\dfrac{19}{30}$
