1. Tính nhanh
a) $\frac{1}{30}$+$\frac{1}{42}$+$\frac{1}{56}$+$\frac{1}{72}$+$\frac{1}{90}$
= $\frac{1}{5.6}$+$\frac{1}{6.7}$+$\frac{1}{7.8}$+$\frac{1}{8.9}$+$\frac{1}{9.10}$
= $\frac{1}{5}$-$\frac{1}{6}$+$\frac{1}{6}$-$\frac{1}{7}$+$\frac{1}{7}$-$\frac{1}{8}$+$\frac{1}{8}$-$\frac{1}{9}$+$\frac{1}{9}$-$\frac{1}{10}$
= $\frac{1}{5}$-$\frac{1}{10}$
= $\frac{1}{10}$
2, Tính
a) (2-$\frac{2}{5}$):$\frac{14}{15}$
= $\frac{8}{5}$:$\frac{14}{15}$
= $\frac{12}{7}$
b) $\frac{11}{4}$+$\frac{5}{3}$:$\frac{5}{2}$
= $\frac{11}{4}$+$\frac{2}{3}$
= $\frac{41}{12}$
