Bài 102: Tính bằng cách thuận tiện nhất
a) 2/9+1/5+7/9+4/5 \(=\left(\dfrac{2}{9}+\dfrac{7}{9}\right)+\left(\dfrac{1}{5}+\dfrac{4}{5}\right)=\dfrac{9}{9}+\dfrac{5}{5}=1+1=2\)
b) 1/12+3/16+5/12+5/16 \(=\left(\dfrac{1}{12}+\dfrac{5}{12}\right)+\left(\dfrac{3}{16}+\dfrac{5}{16}\right)=\dfrac{6}{12}+\dfrac{8}{16}=\dfrac{1}{2}+\dfrac{1}{2}=\dfrac{2}{2}=1\)
3/4+2/3=\(\dfrac{3.3+2.4}{4.3}=\dfrac{9+8}{12}=\dfrac{17}{12}\) 3/5+7/10=\(\dfrac{3.2+7}{10}=\dfrac{13}{10}\) 1/4+7/6=\(\dfrac{6+7.4}{24}=\dfrac{6+28}{24}=\dfrac{17}{12}\)
2/3-2/7=\(\dfrac{2.7-2.3}{21}=\dfrac{14-6}{21}=\dfrac{8}{21}\) 3/4-5/12=\(\dfrac{3.3-5}{4.3}=\dfrac{1}{3}\) 5/6-2/9=\(\dfrac{5.9-2.6}{54}=\dfrac{45-12}{54}=\dfrac{33}{54}=\dfrac{11}{18}\)
2+5/7=\(\dfrac{2.7+5}{7}=\dfrac{14+5}{7}=\dfrac{29}{7}\)
13/5-2=\(\dfrac{13-2.5}{5}=\dfrac{13-10}{5}=\dfrac{3}{5}\)
3-3/8=\(\dfrac{3.8-3}{8}=\dfrac{24-3}{8}=\dfrac{21}{8}\)
1/2+1/3+1/6=\(\dfrac{6+4+2}{12}=\dfrac{12}{12}=1\)
5/12+5/6-3/4\(=\dfrac{5+5.2-3.3}{12}=\dfrac{5+10-9}{12}=\dfrac{6}{12}=\dfrac{1}{2}\)
1-(1/5+1/2)=\(1-\left(\dfrac{2+5}{10}\right)=1-\dfrac{7}{10}=\dfrac{10-7}{10}=\dfrac{3}{10}\)
