Ta có: 1.3.5....39/21.22.23...40
=1.3.5....19/22.24.26....40
=1.3.5.7....19/2.11.2.12.2.13.....2.20
=1.3.5.7....19/2^10.11.12.13.....20
=1.3.5.7.9/2.^10.12.14.16.18.20
=1.3.5.7.9/2^10.2.6.2.7.2.8.2.9.2.10
=1.3.5.7.9/2^15.6.7.8.9.10
=1.3.5/2^15.6.8.10
=1.3.5/2^15.2.3.2.4.2.5
=1.3.5/2^18.3.4.5
=1/2^18.4
=1/2^18.2^2
=1/2^20
Vậy 1.3.5. . . . . 39/21.22.23. . . . 40=1/2^20