Tính bằng cách hợp lí: B = 1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42 + 1/56

B=12+16+112+120+130+142+156+172+190

B=11.2+12.3+13.4+14.5+15.6+16.7+17.8+18.9+19.10

=1−12+12−13+13−14+14−15+...+19−110

=1+−12+12+−13+13+−14+14+−15+15+...+−19+19−110

=1−110=910