A=5+52+53+54+.......+519+520
=(5+52)+(53+54)+...+(519+520)
=1.(5+52)+52.(5+52)+...+518.(5+52)
=1.30+52.30+...+518.30
=30(1+52+...+518) chia hết cho 30
=>dpcm
Ta có: A=(5+52)+(53+54)+.............+(519+520)
A=1.(5+52)+ 52.(5+52)+..........+518.(5+52)
A= 30.(1+52+............+518)
=> A chia hết cho 30 ĐPCM
Ta có (5+52) chia hết cho 30
(53+54) chia hết cho 30
................................
(519+520) chia hết cho 30
=>A chia hết cho 30
A=5+5^2+5^3+5^4+...+5^19+5^20
=(5+5^2)+5^2(5+5^2)+...+5^18(5+5^2)
=(5+5^2)(1+5^2+...+5^18)
=30(1+5^2+...+5^18) chia hết cho 30
A=5+52+53+54+...+519+520
=(5+52)+(53+54)+...+(519+520)
=1.(5+52)+52.(5+52)+...+518.(5+52)
=1.30+52.30+...+518.30
=30.(1+52+...+518) chia hết cho30
suy ra dpcm
