Giải câu 4 bài Luyện tập sgk Toán 4 trang 131

a) Rút gọn

\(\frac{3}{15}\)= \(\frac{3 : 3}{15 : 3}\)= \(\frac{1}{5}\)  

\(\frac{5}{35}\) =  \(\frac{5 : 5}{35 : 5}\) = \(\frac{1}{7}\)

Trừ hai phân số:

\(\frac{3}{15}-\frac{5}{35}= \frac{1}{5}-\frac{1}{7}= \frac{1 \times 7}{5 \times 7}-\frac{1 \times 5}{7 \times 5} = \frac{7}{35}-\frac{5}{35}=\frac{2}{35}\)

b) Rút gọn;

\(\frac{18}{27}\)= \(\frac{18 : 9}{27 : 9}\)= \(\frac{2}{3}\)

\(\frac{2}{6}\)= \(\frac{2 : 2}{6 : 2}\)= \(\frac{1}{3}\)

Trừ hai phân số:

\(\frac{18}{27}-\frac{2}{6}= \frac{2}{3}-\frac{1}{3}=\frac{1}{3}\)

c) Rút gọn:

\(\frac{15}{25}\) = \(\frac{15 : 5}{25 : 5}\) = \(\frac{3}{5}\)

\(\frac{3}{21}\) = \(\frac{3 : 3}{21 :3}\) = \(\frac{1}{7}\)

Trừ hai phân số:

\(\frac{15}{25}-\frac{3}{21}= \frac{3}{5}-\frac{1}{7}= \frac{3 \times 7}{5 \times 7}-\frac{1 \times 5}{7 \times 5}= \frac{21}{35}-\frac{5}{35}=\frac{16}{35}\)

d) Rút gọn:

\(\frac{24}{36}\) = \(\frac{24 : 12}{36 : 12}\) = \(\frac{2}{3}\)

\(\frac{6}{12}\)= \(\frac{6 : 6}{12 : 6}\)= \(\frac{1}{2}\)

\(\frac{24}{36}-\frac{6}{12}= \frac{2}{3}-\frac{1}{2}= \frac{4}{6}-\frac{3}{6}=\frac{1}{6}\)