1. a) BCNN(12, 15) = 60, chọn mẫu chung là 60.
Thừa số phụ: 60 : 12 = 5; 60 : 15 = 4.
\(\frac{5}{12} = \frac{5 \times 5}{12 \times 5} = \frac{25}{60}\)
\(\frac{7}{15} = \frac{7 \times 4}{15 \times 4} = \frac{28}{60}\)
b) BCNN(7, 9, 12) = 252, chọn mẫu chung là 252.
Thừa số phụ: 252 : 7 = 36; 252 : 9 = 28; 252 : 12 = 21.
\(\frac{2}{7} = \frac{2 \times 36}{7 \times 36} = \frac{72}{252}\)
\(\frac{4}{9} = \frac{4 \times 28}{9 \times 28} = \frac{112}{252}\)
\(\frac{7}{12} = \frac{7 \times 21}{12 \times 21} = \frac{147}{252}\)
2. a) BCNN(8, 24) = 24.
\(\frac{3}{8} + \frac{5}{24} = \frac{3 \times 3}{8 \times 3} + \frac{5}{24} = \frac{9}{24} + \frac{5}{24} = \frac{14}{24} = \frac{7}{12}\)
b) BCNN(16, 12) = 48.
\(\frac{7}{16} - \frac{5}{12} = \frac{7 \times 3}{16 \times 3} - \frac{5 \times 4}{12 \times 4} = \frac{21}{48} - \frac{20}{48} = \frac{1}{48}\)
