Nếu thay dấu (.) thành dấu (+) thì làm như sau:
A=5+52+53+...+520
=(5+52)+(53+54)+...+(519+520)
=50.5.(1+5)+52.5.(1+5)+...+518.5.(1+5)
=50.5.6+52.5.6+...+518.5.6
=50.30+52.30+...+518.30
=30.(50+52+...+518) chia het cho 30 => So do la boi cua 30
=> 5+52+53+...+520 la boi cua 30 (ĐPCM0