2.57. Trang 56

a) Ta có: 12=$2^{2}$.3; 16=$2^{4}$ nên BCNN(12,16)=$2^{4}$.3=48 nên chọn MSC=48.

$\dfrac{5}{12}$+$\dfrac{3}{16}$=$\dfrac{5.4}{12.4}$+$\dfrac{3.3}{16.3}$=$\dfrac{20}{48}$+$\dfrac{9}{48}$=$\dfrac{29}{48}$

b) Ta có: 15=3.5; 9=$3^{2}$ nên BCNN(15,9)=$3^{2}$.5=45 nên chọn MSC=45.

$\dfrac{4}{15}$-$\dfrac{2}{9}$=$\dfrac{4.3}{15.3}$-$\dfrac{2.5}{9.5}$=$\dfrac{12}{45}$-$\dfrac{10}{45}$=$\dfrac{2}{45}$