D:2=1/12+1/20+1/30+1/42+1/72
=1/3-1/4=1/4-1/5+1/5-1/6+1/6-1/7+1/8-1/9
=1/3-1/7+1/8-1/9
Bạn tự tính nốt nhé
bạn thiếu phân số \(\frac{1}{28}\)
D = \(\frac{2}{12}\)+ \(\frac{2}{20}\)+\(\frac{2}{30}\)+\(\frac{2}{42}\)+ \(\frac{2}{56}\)+\(\frac{2}{72}\)
D : 2 = \(\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}\)
D : 2 = \(\frac{1}{3}-\frac{1}{9}=\frac{2}{9}\)=> D = \(\frac{4}{9}\)