1.tính nhanh a,1/30+1/42+1/56+1/72+1/90 2,tính a,(2-2/5):14/15 b,11/4+5/3:5/2

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) $\frac{11}{4}$+$\frac{5}{3}$:$\frac{5}{2}$ 

=  $\frac{11}{4}$+$\frac{2}{3}$

=  $\frac{41}{12}$