A = 4 + 42 + 43 + ... + 42013
A = (4 + 42) + (43 + 44) + ... + (42012 + 42013)
A = (4 + 42) + 42(4 + 42) + ... + 42011(4 + 42)
A = 20 +(42.20)+ ... +(42011.20)
A = 20(1 + 42 + ... + 42011) ⋮ 5
➤ A ⋮ 5
A = 4 + 42 + 43 + ... + 42013
A =(4 + 42 + 43) + (44 + 45 + 46 )+ ...+ (42011 + 42012 + 42013)
A= (4 + 42 + 43) +43(4 + 42 + 43) + ... +42010(4 + 42 + 43)
A = 81 + (43.81) + ... +(42010.81)
A = 81(1 + 43 + ... + 42010) ⋮ 21
➤ A ⋮ 21
A=1+32+34+36+....+3100
=>9A=32+34+36+38+....+3102
=>8A=3102-1
=>A=3102-1/8
b)A=1+53+56+59+.....+599
125A=53+56+59+512+.....+5102
124A=5102-1
A=5102-1/124
BT3:
1+4+42+43+...+458+459
=>(1+4)+(42+43)+...........+(458+459)chia hết cho 5
=>5+42.5+...........+458.5 chia hết cho 5
2)1+4+42+43+........+458+459
=>(1+4+42)+(43+44+45)+..........+(457+458+459)
=>21+43.21+........+457.24 chia hết cho 21
3)1+4+42+43+..........+458+459
=>(1+4+42+43)+(44+45+46+47)+............+(456+457+458+459)
=>85+44.85+..........+456.85 chia hết cho 85
4)5+53+55+.........+5202+5203 ( đề sai vì ta thấy 53 tới 55 mà 5202 tới 5203)
