a, Chứng minh rằng A chia hết cho 3 

A = 2 + 2² + 2³ + .....+ 2⁶⁰ 

A = ( 2+2² ) + (2³ + 2⁴ ) + .....+ (2⁵⁹ + 2⁶⁰ )

A  = 2(1+2 ) + 2³(1+2) +,...+  2⁵⁹(1+2)

A = 2.3 + 2³.3 +  ....+2⁵⁹.3 

A = 3(2+2³+....+2⁵⁹ ) \(⋮3\) 

=> đpcm 

chứng minh ằng A chia hết cho 7 

A = 2+2² + 2³ + .....+ 2⁶⁰

A = ( 2+2² + 2³ ) + (2⁴ + 2⁵ + 2⁶) + .... + (2⁵⁸+2⁵⁹+2⁶⁰)

A = 2(1+2 +2² ) +2⁴ (1+2 +2² ) + .... +2⁵⁸(1+2 +2² )

A = 2.7 +2⁴.7  + ....+2⁵⁸.7 

A= 7(2+2⁴ ....+2⁵⁸ )\(⋮7\)

=> đpcm

Chứng minh A chia hết cho 15 

A = 2 + 2² + 2³ + .....+ 2⁶⁰ 

A = ( 2 + 2² + 2³ +2⁴ ) +....+  (2⁵⁷ + 2⁵⁸ + 2⁵⁹ + 2⁶⁰ ) 

A = 2(1+2+2² + 2³ ) + .....+ 2⁵⁷(1+2+2²+2³)

A = 2.15 + ....+ 2⁵⁷.15

A = 15.(2+...+2⁵⁷) \(⋮15\) 

=> đpcm  

b,

chứng minh chia hết cho 13

 B= 3 + 3³ + 3⁵ + +  ..........+ 3¹⁹⁹¹ 

B = (3+3³ + 3⁵ ) + (3⁷  + 3⁹ +3¹¹ ) + ......+ (3¹⁹⁸⁷ + 3¹⁹⁸⁹ + 3¹⁹⁹¹ ) 

B = 3(1+3² +3⁴ ) + 3⁷(1+3² + 3⁴ ) + ....+ 3¹⁹⁸⁷(1+3² + 3⁴ )

B = 3.91 + 3⁷.91 + ...+ 3¹⁹⁸⁷.91 

B = 91(3+3⁷ + ... 3¹⁹⁸⁷ ) 

B = 7.13.(3+3⁷ + ... 3¹⁹⁸⁷ )  \(⋮13\) 

=> đpcm 

chứng minh chia hết cho 41 

B = 3+3³ + 3⁵ + ...+ 3¹⁹⁹¹

B = (3+3³ + 3⁵  + 3⁷ ) + ...(3¹⁹⁸⁵ + 3¹⁹⁸⁷ + 3¹⁹⁸⁹ + 3¹⁹⁹¹  ) 

B = 3(1+3² + 3⁴ + 3⁶ ) + ...+ 3¹⁹⁸⁵(1+3² + 3⁴ + 3⁶)

B = 3. 820 + ...+ 3¹⁹⁸⁵.820

B = 820(3+...+3¹⁹⁸⁵)

B = 20.41 (3+...+3¹⁹⁸⁵) \(⋮41\) 

=> đpcm

a là hợp số;  b là hợp số ;  c là hợp số; k cho tui nha 

Mong các bn giải chi tiết giúp mik nha

D.M là trung điểm OA

cảm ơn bạn nha mình đang làm bài tập thầy giao nhưng khó quá nè 

- Chịu tìm hiểu 1 chút !

Hello ! Today I will guide you to travel to Mount Phan - Xi - Pang . First, let's find out about it ! Fansipan, also known as Fansipan, is the highest mountain of the Indochinese peninsula (3,143 m), located in the center of Hoang Lien Son range. 9 km southwest of Sapa town. To conquer the top of Phan Xi Pang used to take 5 to 6 days, now that time is shortened to only about 2 to 3 days. To be able to join this journey you need to have a good health, a full set of essential items and an indispensable element is the street people. Conquering the top of Phan Xi Pang becomes a burning desire for all who love adventure and adventure. On the way to conquer the top of Phan Xi Pang, visitors also have the opportunity to explore the wonderful nature of the Hoang Lien range. At an altitude of 2,400m, you feel the wind and clouds blend with the forest trees, spreading your hands as if it could be five clouds. From the height of 2,800m, the clouds suddenly disappeared, the sky was clear and clear. Only the wind blows the vegetation to sk to the rock. The next climb you will see a giant rock laid out by small rocks a table. Phan Xi Pang Peak! People call it "Hua Si pan" which means big rock. So you have completed the amazing adventure.

My parents gave me a trip to Sapa as a present for my effort in the previous school year and the trip was an unforgettable memory to me. My family got there in the morning when the weather was cool but it began to become warmer at noon and in the afternoon but in the evening, the outside was quite cold and even having snow at night. It was such an amazing experience that I really enjoyed it. We also went sightseeing and the natural beauty here took my breath away. The roman and peaceful atmosphere with ranges of mountains surrounded by fog soon made a strong impression on me. I also had an opportunity to try seating on a cable car and enjoyed how breathtaking the view was.That was the first time in my life that I could see mountain, forests and waterfalls with my own eyes.The next day, my mom and I spent time going to the market where many of activities took place and I bought some souvenirs as gifts for my close friend. Especially, I was totally surprised at the ability of speaking English of local residents, especially small children. Besides, I also had a chance to try local food such as Sapa Cap Nach Pork, bamboo sky rice, Tao Meo Wine… Each has its unique flavors that all satisfied my tastes. Finally my family had to say goodbye to Sapa but I still hope to come back here one day in a nearest future.

câu a) 111....1 (2019 số 1) chia hết cho 11 vì có các chữ số giống nhau

câu b) tương tự nha

c) 1112111 chia hết cho 11011 ( dựa vào dấu hiệu nhận biết của các chữ số )

d) tương tự

câu a,b đều chia hết cho 1, 11 và chính nó => hợp số

câu c chia hết cho 101 còn câu d chia hết chi 1111=> hợp số

câu e) -.- đang tắc