Câu 3: Trang 60 sách VNEN toán lớp 4Tính bằng hai cách:$\left ( \frac{1}{4}+\frac{1}{7} \right...

a. Cách 1:
$\left ( \frac{1}{4}+\frac{1}{7} \right )\times \frac{1}{3}=\left ( \frac{7}{28}+\frac{4}{28} \right )\times \frac{1}{3}=\frac{11}{28}\times \frac{1}{3}=\frac{11}{84}$
$\left ( \frac{1}{4}+\frac{1}{7} \right )\times \frac{1}{3}=\frac{1}{4}\times \frac{1}{3}+\frac{1}{7}\times \frac{1}{3}=\frac{1}{12}+\frac{1}{21}=\frac{7}{84}+\frac{4}{84}=\frac{11}{84}$

b. Cách 2:
$\left ( \frac{1}{4}-\frac{1}{7} \right )\times \frac{1}{3}=\left ( \frac{7}{28}-\frac{4}{28} \right )\times \frac{1}{3}=\frac{3}{28}\times \frac{1}{3}=\frac{3}{84}$
$\left ( \frac{1}{4}-\frac{1}{7} \right )\times \frac{1}{3}=\frac{1}{4}\times \frac{1}{3}-\frac{1}{7}\times \frac{1}{3}=\frac{1}{12}-\frac{1}{21}=\frac{7}{84}-\frac{4}{84}=\frac{3}{84}$

Đáp án:
a.
$\left ( \frac{1}{4}+\frac{1}{7} \right )\times \frac{1}{3}=\frac{11}{84}$

b.
$\left ( \frac{1}{4}-\frac{1}{7} \right )\times \frac{1}{3}=\frac{3}{84}$