Giải câu 3 trang 137 toán tiếng anh lớp 4

Giải câu 3 trang 137 toán tiếng anh lớp 4.

a) $(\frac{1}{3}+\frac{1}{5})\times \frac{1}{2}$

Solution 1:

Cách 1:

$(\frac{1}{3}+\frac{1}{5})\times \frac{1}{2}=(\frac{1\times 5}{3\times 5}+\frac{1\times 3}{5\times 3})\times \frac{1}{2}$ ( make the fractions have a common denominoator) (quy đồng mẫu số)

$=(\frac{5}{15}+\frac{3}{15})\times \frac{1}{2}=\frac{8}{15}\times \frac{1}{2}=\frac{8\times 1}{15\times 2}=\frac{4\times 2\times 1}{15\times 2}=\frac{4}{15}$(both numerator and denominator are simplified for 2) (rút gọn cả tử và mẫu cho 2)

Solution 2:

Cách 2:

$(\frac{1}{3}+\frac{1}{5})\times \frac{1}{2}=\frac{1}{3}\times \frac{1}{2}+\frac{1}{5}\times \frac{1}{2}=\frac{1\times 1}{3\times 2}+\frac{1\times 1}{5\times 2}=\frac{1}{6}+\frac{1}{10}=\frac{1\times 10}{6\times 10}+\frac{1\times 6}{10\times 6}$ ( make the fractions have a common denominoator) (quy đồng mẫu số)

$=\frac{10}{60}+\frac{6}{60}=\frac{16}{60}=\frac{16:4}{60:4}=\frac{4}{15}$

b) $(\frac{1}{3}-\frac{1}{5})\times \frac{1}{2}$

Solution 1:

Cách 1:

$(\frac{1}{3}-\frac{1}{5})\times \frac{1}{2}=(\frac{1\times 5}{3\times 5}-\frac{1\times 3}{5\times 3})\times \frac{1}{2}$

$=(\frac{5}{15}-\frac{3}{15})\times \frac{1}{2}=\frac{2}{15}\times \frac{1}{2}=\frac{2\times 1}{15\times 2}=\frac{1}{15}$(both numerator and denominator are simplified for 2) (rút gọn cả tử và mẫu cho 2)

Solution 2:

Cách 2:

$(\frac{1}{3}-\frac{1}{5})\times \frac{1}{2}=\frac{1}{3}\times \frac{1}{2}-\frac{1}{5}\times \frac{1}{2}=\frac{1\times 1}{3\times 2}-\frac{1\times 1}{5\times 2}=\frac{1}{6}-\frac{1}{10}=\frac{1\times 10}{6\times 10}-\frac{1\times 6}{10\times 6}$

$=\frac{10}{60}-\frac{6}{60}=\frac{4}{60}=\frac{4:4}{60:4}=\frac{1}{15}$