$\dfrac{1 \times 3 \times 5 + 2 \times 6 \times 10 + 4 \times 12 \times 20 + 7 \times 21 \times 35}{1 \times 5 \times 7 + 2 \times 10 \times 14 + 4 \times 20 \times 28 + 7 \times 35 \times 49}$

$=$ $\dfrac{1 \times 3 \times 5 + 2 \times 3 \times 2 \times 5 \times 2 + 4 \times 3 \times 4 \times 5 \times 4 + 7 \times 3 \times 7 \times 5 \times 7}{1 \times 5 \times 7 + 2 \times 5 \times 2 \times 7 \times 2 + 4 \times 5 \times 4 \times 7 \times 5 + 7 \times 5 \times 7 \times 7 \times 7}$

$=$ $\dfrac{3 \times 5 \times (1 + 2 \times 2 \times 2 + 4 \times 4 \times 4 + 7 \times 7 \times 7)}{5 \times 7 \times ( 1 + 2 \times 2 \times 2 + 4 \times 4 \times 4 + 7 \times 7 \times 7)}$

$=$ $\dfrac{3}{7}$

Đáp án: `624/1465`

Giải thích các bước giải:

$\dfrac{1 \times 3 \times 5 + 2 \times 6 \times 10 + 4 \times 12 \times 20 + 7 \times 21 \times 35}{1 \times 5 \times 7 + 2 \times 10 \times 14 + 4 \times 20 \times 28 + 7 \times 35 \times 49}$ `= 6240/14560 = 39/91`