A = 20 + 21 + 23 + 24 + ... + 219 + 220
A = ( 20 + 2 ) + ( 23 + 24 ) + ... + ( 219 + 220 )
A =3 + 23 . ( 1 + 2 ) + ... + 219 . ( 1 + 2 )
A = 3 + 23 . 3 + .... + 219 . 3
A = 3 . ( 1 + 23 + ... + 219 ) \(⋮\)3
A = 73 + 74 + ... + 798
A = ( 73 + 74 ) + ... + ( 797 + 798 )
A = 73 . ( 1 + 7 ) + .... + 797 . ( 1 + 7 )
A = 73 . 8 + ... + 797 . 8
A = 8 . ( 73 + ... + 797 ) \(⋮\)8
1, đề sai , đề phải là : A = 2^1+2^2+2^3+2^4+.....+2^19+2^20
A = (2^1+2^2)+(2^3+2^4)+....+(2^19+2^20)
= 2.(1+2)+2^3.(1+2)+.....+2^19.(1+2)
= 2.3+2^3.3+.....+2^19.3
= 3.(2+2^3+.....+2^19) chia hết cho 3
2, A = (7^3+7^4)+(7^5+7^6)+.....+(7^97+7^98)
= 7^3.(1+7)+7^5.(1+7)+.....+7^97.(1+7)
= 7^3.8+7^5.8+.....+7^97.8
= 8.(7^3+7^5+.....+7^97) chia hết cho 8
Tk mk nha
1) Ta có :
A = 20 + 21 + 23 + 24 + ... + 219 + 220
= 1 + 2 + 23 + 24 + ... + 219 + 220
= (1 + 2) + (23 + 24) + ... + (219 + 220)
= 3 + 22. (2 + 22) + ... + 218. (2 + 22)
= 3 + 22 . 6 + ... + 218 . 6 chia hết cho 3
2) Ta có :
A = 73 + 74 + 75 + ... + 798
= (73 + 74) + (75 + 76) + ... + (797 + 798)
= 73.(1 + 7) + 75.(1 + 7) + ... + 797.(1 + 7)
= 73 .8 + 75 . 8 + ... + 797 . 8
= (73 + 75 + ... + 797) . 8 chia hết cho 8