Ta có : $\frac{31}{2}$ . $\frac{32}{2}$ . $\frac{33}{2}$ ..... $\frac{60}{2}$
= $\frac{31.32.33.....60}{2^{20}}$
= $\frac{( 1.2.3.....30) . (31.32.33.....60}{(1.2.3.....30)2^{20}}$
= $\frac{1.2.3.....60}{2.4.6.....60}$
= $\frac{(1.3.5.....59).(2.4.6.....60)}{2.4.6.....60}$
= 1 . 3 . 5 ..... 59
Vậy (đpcm)
