Bài 78: Luyện tập

a: \(\dfrac{1}{4}+\dfrac{3}{8}=\dfrac{2}{8}+\dfrac{3}{8}=\dfrac{5}{8}\)

b: \(\dfrac{5}{12}+\dfrac{7}{3}=\dfrac{5}{12}+\dfrac{28}{12}=\dfrac{5+28}{12}=\dfrac{33}{12}=\dfrac{11}{4}\)

c: \(\dfrac{3}{16}+\dfrac{9}{4}=\dfrac{3}{16}+\dfrac{36}{16}=\dfrac{3+36}{16}=\dfrac{39}{16}\)

d: \(\dfrac{9}{14}-\dfrac{4}{7}=\dfrac{9}{14}-\dfrac{8}{14}=\dfrac{9-8}{14}=\dfrac{1}{14}\)

e: \(\dfrac{2}{5}-\dfrac{4}{15}=\dfrac{6}{15}-\dfrac{4}{15}=\dfrac{6-4}{15}=\dfrac{2}{15}\)

g: \(\dfrac{8}{7}-\dfrac{10}{21}=\dfrac{24}{21}-\dfrac{10}{21}=\dfrac{24-10}{21}=\dfrac{14}{21}=\dfrac{2}{3}\)

a: \(\dfrac{8}{18}+\dfrac{5}{3}=\dfrac{4}{9}+\dfrac{5}{3}=\dfrac{4}{9}+\dfrac{15}{9}=\dfrac{4+15}{9}=\dfrac{19}{9}\)

b: \(\dfrac{8}{24}+\dfrac{4}{48}=\dfrac{1}{3}+\dfrac{1}{12}=\dfrac{4}{12}+\dfrac{1}{12}=\dfrac{4+1}{12}=\dfrac{5}{12}\)

c: \(\dfrac{20}{15}-\dfrac{4}{45}=\dfrac{4}{3}-\dfrac{4}{45}=\dfrac{60}{45}-\dfrac{4}{45}=\dfrac{60-4}{45}=\dfrac{56}{45}\)

d: \(\dfrac{40}{32}-\dfrac{1}{2}=\dfrac{5}{4}-\dfrac{1}{2}=\dfrac{5-2}{4}=\dfrac{3}{4}\)

a: \(5+\dfrac{3}{2}=\dfrac{10}{2}+\dfrac{3}{2}=\dfrac{10+3}{2}=\dfrac{13}{2}\)

\(\dfrac{3}{4}+2=\dfrac{3}{4}+\dfrac{8}{4}=\dfrac{3+8}{4}=\dfrac{11}{4}\)

\(\dfrac{8}{9}+3=\dfrac{8}{9}+\dfrac{27}{9}=\dfrac{8+27}{9}=\dfrac{35}{9}\)

b: \(1-\dfrac{1}{2}=\dfrac{2}{2}-\dfrac{1}{2}=\dfrac{2-1}{2}=\dfrac{1}{2}\)

\(5-\dfrac{7}{3}=\dfrac{15}{3}-\dfrac{7}{3}=\dfrac{15-7}{3}=\dfrac{8}{3}\)

\(\dfrac{11}{2}-3=\dfrac{11}{2}-\dfrac{6}{2}=\dfrac{11-6}{2}=\dfrac{5}{2}\)

báo gấm chạy nhanh hơn:

\(\dfrac{1}{2}-\dfrac{1}{6}=\dfrac{3}{6}-\dfrac{1}{6}=\dfrac{2}{6}=\dfrac{1}{3}\)(quãng đường)

=>Chọn D

Khối lượng nguyên liệu mẹ đã mua là:

\(\dfrac{9}{4}+\dfrac{1}{2}+\dfrac{3}{4}=\dfrac{9}{4}+\dfrac{3}{4}+\dfrac{2}{4}=\dfrac{9+3+2}{4}=\dfrac{14}{4}=3,5\left(kg\right)\)

\(\dfrac{8}{9}+\dfrac{1}{3}=\dfrac{8}{9}+\dfrac{3}{9}=\dfrac{11}{9}\)

\(\dfrac{3}{4}-\dfrac{1}{2}=\dfrac{3}{4}-\dfrac{2}{4}=\dfrac{1}{4}\)

\(\dfrac{7}{18}-\dfrac{1}{3}=\dfrac{7}{18}-\dfrac{6}{18}=\dfrac{1}{18}\)

\(\dfrac{2}{3}-\dfrac{7}{12}=\dfrac{8}{12}-\dfrac{7}{12}=\dfrac{1}{12}\)

\(\dfrac{4}{6}+\dfrac{1}{12}=\dfrac{2}{3}+\dfrac{1}{12}=\dfrac{8}{12}+\dfrac{1}{12}=\dfrac{9}{12}=\dfrac{3}{4}\)

\(\dfrac{19}{20}-\dfrac{3}{4}=\dfrac{19}{20}-\dfrac{15}{20}=\dfrac{4}{20}=\dfrac{1}{5}\)

\(\dfrac{9}{12}+\dfrac{3}{4}=\dfrac{3}{4}+\dfrac{3}{4}=\dfrac{6}{4}=\dfrac{3}{2}\)

\(\dfrac{1}{5}+\dfrac{2}{10}=\dfrac{1}{5}+\dfrac{1}{5}=\dfrac{2}{5}\)

\(\dfrac{4}{5}-\dfrac{2}{15}=\dfrac{12}{15}-\dfrac{2}{15}=\dfrac{10}{15}=\dfrac{2}{3}\)

\(\dfrac{9}{18}+\dfrac{1}{2}=\dfrac{1}{2}+\dfrac{1}{2}=1\)