Giải thích các bước:
Bài 1:
$\frac{12}{30}$ = $\frac{12:6}{30:6}$ = $\frac{2}{5}$
$\frac{20}{45}$ = $\frac{20x5}{45:5}$ = $\frac{4}{9}$
$\frac{28}{70}$ = $\frac{28:14}{70:14}$ = $\frac{2}{5}$
$\frac{34}{51}$ = $\frac{34:17}{51:17}$ = $\frac{2}{3}$
Bài 2:
$\frac{5}{18}$ = $\frac{5}{18}$ ( loại )
$\frac{6}{27}$ = $\frac{6:3}{27:3}$ = $\frac{2}{9}$ ( thỏa mãn )
$\frac{14}{63}$ = $\frac{14:7}{63:7}$ = $\frac{2}{9}$ ( thỏa mãn )
$\frac{10}{36}$ = $\frac{10:5}{36:5}$ = $\frac{2}{7,2}$ ( loại )
đáp án:
$\frac{6}{27}$ và $\frac{14}{63}$
Bài 3:
a) $\frac{4}{3}$ = $\frac{4x8}{3x8}$ = $\frac{32}{24}$
$\frac{5}{8}$ = $\frac{5x3}{8x3}$ = $\frac{15}{24}$
b) $\frac{4}{5}$ = $\frac{4x9}{5x9}$ = $\frac{36}{45}$
$\frac{5}{9}$ = $\frac{5x5}{9x5}$ = $\frac{25}{45}$
c) $\frac{4}{9}$ = $\frac{4x4}{9x4}$ = $\frac{16}{36}$
$\frac{7}{12}$ = $\frac{7x3}{12x3}$ = $\frac{21}{36}$
d) $\frac{1}{2}$ = $\frac{1x6}{2x6}$ = $\frac{6}{12}$
$\frac{2}{3}$ = $\frac{2x4}{3x4}$ = $\frac{8}{12}$
$\frac{7}{12}$ = $\frac{7x1}{12x1}$ = $\frac{7}{12}$
