So sánh:
C= 1 . 3 . 5 . ... .99
và
D= 51/2 . 52/2 . ... . 100/2
$C . (2.4.6.8....100) = 1.2.3.4...100\\
C = \dfrac{1.2.3.4...100}{2.4.6.8....100} \\
= \dfrac{1.2.3.4...100}{2.1.2.2.2.3.2.4....2.50} \\
= \dfrac{1.2.3.4...100}{2^{50}.(1.2.3.4...50)}\\
= \dfrac{51 .52.53.....100}{2^{50}}\\
= \dfrac{51}{2} .\dfrac{52}{2}.\dfrac{53}{2} ....\dfrac{100}{2} = D$
Vậy ...