\(B=2+2^2+2^3+2^4+...+2^{99}+2^{100}=2\left(1+2^2+2^3+2^4\right)+...+2^{96}\left(1+2^2+2^3+2^4\right)=2.31+2^6.31+...+2^{96}.31=31\left(2+2^6+...+2^{96}\right)⋮31\)

B=2+22+23+24+...+299+2100=2(1+22+23+24)+...+296(1+22+23+24)=2.31+26.31+...+296.31=31(2+26+...+296)⋮31

Mình đã làm như sau:

A=2⁹⁸+2²-2⁹⁸+2⁹⁴+2²-2⁹⁴+…+2²+2²-2²

=2²+2²+…+2² = 4+4+…+4

ð Mỗi số hạng trong tổng đều chia hết cho 4 thì => tổng chia hết cho 4

ð A chia hết cho 4

Nhưng bé nhà mình cứ kêu đúng nhưng ko giống cách làm của học sinh lớp 6

Vậy OLM vui lòng cho mình hỏi còn cách làm nào phù hợp với học sinh lớp 6 không ah?

*Sửa lại đề*

A = 2¹+ 2²+ 2³+ 2⁴ + .. + 2¹⁰⁰

A = (2¹+2²) + (2³+ 2⁴) +...+ (2⁹⁹+ 2¹⁰⁰)

A = 2.(1+2) + 2³.(1+2) + .. + 2⁹⁹. (1+2)

A = 2.3 + 2³. 3 + .. + 2⁹⁹.3

A = 3 . (2¹ + 2³ + .... + 2⁹⁹)

Mà 3 chia hết cho 3 

=> A chia hết cho 3

\(B=2+2^2+2^3+...+2^{100}\)

\(\Rightarrow2B=2^2+2^3+2^4+...+2^{101}\)

\(\Rightarrow2B-B=2^2+2^3+2^4+...+2^{101}-2-2^2-2^3-...-2^{100}\)

\(\Rightarrow2B-B=2^{101}-2\)

bài 1

2¹⁰¹ - 2

\(A=1+3+3^2+3^3+...+3^{99}\\ \Rightarrow A=\left(1+3+3^2+3^3\right)+\left(3^4+3^5+3^6+3^7\right)+...+\left(3^{96}+3^{97}+3^{98}+3^{99}\right)\)

\(\Rightarrow A=\left(1+3+3^2+3^3\right)+3^4\left(1+3+3^2+3^3\right)+...+3^{96}\left(1+3+3^2+3^3\right)\)

\(\Rightarrow A=\left(1+3+3^2+3^3\right)\left(1+3^4+...+9^{96}\right)\)

\(\Rightarrow A=40\left(1+3^4+...+9^{96}\right)⋮40\)

\(A=1+2+2^2+2^3+2^4+...+2^{99}+2^{100}\)

\(\Rightarrow2A=2+2^2+2^3+2^4+2^5+...+2^{100}+2^{101}\)

\(\Rightarrow2A-A=2^{101}-1\)

\(\Leftrightarrow A=2^{101}-1\)

Đặt biểu thức là A

ta có 2A-A=2^101-1

Đặt \(A=1+2+2^2+2^3+2^4+...+2^{99}+2^{100}\)

\(\Rightarrow2A=2+2^2+2^3+...+2^{100}+2^{101}\)

\(\Rightarrow A=2A-A=\left(2+2^2+2^3+2^4+...+2^{101}\right)-\left(1+2+2^2+2^3+...+2^{100}\right)=2^{101}-1\)

A=2+22+23+...+299+2100A=2+22+23+...+299+2100

⇒2A=22+23+24+...+2100+2101⇒2A=22+23+24+...+2100+2101

⇒A=2101−2⇒A=2101−2

B=3+32+33+...+399+3100B=3+32+33+...+399+3100

⇒3B=32+33+34+...+3100+3101⇒3B=32+33+34+...+3100+3101

⇒2B=3101−3⇒2B=3101−3

⇒B=3101−32

a) \(A=1+2+2^2+...+2^{50}\)

\(\Rightarrow2A=2+2^2+...+2^{51}\)

\(\Rightarrow A=2A-A=2+2^2+...+2^{51}-1-2-2^2-...-2^{50}=2^{51}-1\)

b) \(B=1+3+3^2+...+3^{100}\)

\(\Rightarrow3B=3+3^2+...+3^{101}\)

\(\Rightarrow2B=3B-B=3+3^2+...+3^{101}-1-3-3^2-...-3^{100}=3^{101}-1\)

\(\Rightarrow B=\dfrac{3^{101}-1}{2}\)

c) \(C=5+5^2+...+5^{30}\)

\(\Rightarrow5C=5^2+5^3+...+5^{31}\)

\(\Rightarrow4C=5C-C=5^2+5^3+...+5^{31}-5-5^2-...-5^{30}=5^{31}-5\)

\(\Rightarrow C=\dfrac{5^{31}-5}{4}\)

d) \(D=2^{100}-2^{99}+2^{98}-...+2^2-2\)

\(\Rightarrow2D=2^{101}-2^{100}+2^{99}-...+2^3-2^2\)

\(\Rightarrow3D=2D+D=2^{101}-2^{100}+2^{99}-...+2^3-2^2+2^{100}-2^{99}+...+2^2-2=2^{101}-2\)

\(\Rightarrow D=\dfrac{2^{101}-2}{3}\)

1990.1990 -1992.1988

Lời giải:
Đặt $A=1+2^2+2^4+....+2^{100}$

$A=(1+2^2+2^4)+(2^6+2^8+2^{10})+.....+(2^{96}+2^{98}+2^{100})$

$A=(1+2^2+2^4)+2^6(1+2^2+2^4)+....+2^{96}(1+2^2+2^4)$

$=(1+2^2+2^4)(1+2^6+....+2^{96})$

$=21(1+2^6+....+2^{96})\vdots 21$ 

Ta có đpcm.