Bài 3 trang 149 SGK Toán 5

Quy đồng mẫu số các phân số:

a) \(\dfrac{3}{4}\) và \(\dfrac{2}{5}\);                             

b) \(\dfrac{5}{12}\) và \(\dfrac{11}{36}\);

c) \(\dfrac{2}{3}\), \(\dfrac{3}{4}\) và \(\dfrac{4}{5}\).

a)  \(\dfrac{3}{4}\) = \(\dfrac{3 \times 5}{4 \times  5}\) = \(\dfrac{15}{20}\);               

     \(\dfrac{2}{5}\) = \(\dfrac{2 \times  4}{5 \times 4}\) = \(\dfrac{8}{20}\);

b) \(\dfrac{5}{12}\) = \(\dfrac{5 \times 3}{12 \times 3}\) = \(\dfrac{15}{36}\);             

    Giữ nguyên phân số \(\dfrac{11}{36}.\) 

c) \(\dfrac{2}{3}\) = \(\dfrac{2 \times 4 \times 5}{3 \times 4 \times5}\) = \(\dfrac{40}{60}\);             

    \(\dfrac{3}{4}\) = \(\dfrac{3 \times 3 \times 5}{4 \times 3 \times 5}\) = \(\dfrac{45}{60}\);

    \(\dfrac{4}{5}\) = \(\dfrac{4 \times 3 \times 4}{5 \times 3 \times 4}\) = \(\dfrac{48}{60}\).