$\frac{11}{2}$.$\frac{12}{2}$. $\frac{13}{2}$ ...$\frac{20}{2}$
=$\frac{11.12.13...20}{2^{10}}$
=$\frac{11.12.13...20.1.2.3...10}{2^{10}.1.2.3...20}$
=$\frac{1.2.3.4...20}{(1.2).(2.2).(2.3)...(10.2)}$
=$\frac{1.2.3...20}{2.4.6...20}$
=1.3.5...19
Vậy.....