Thấu kính hội tụ thường được làm bằng vật liệu trong suốt, thủy tinh hoặc nhựa chẳng hạn: Thấu kính hội tụ thường có độ dày phần rìa mỏng hơn so với phần giữa.

\(\frac{15^{20}\cdot9^{10}}{27^{12}\cdot25^{10}}=\frac{3^{20}\cdot5^{20}\cdot3^{20}}{3^{36}\cdot5^{20}}=\frac{3^{40}}{3^{36}}=3^4=81\)

Trong thí nghiệm ở hình 42.2 SGK, điểm hội tụ F của chùm tia ló nằm trên trục chính, đó là đường thẳng chứa tia tới ở giữa

thời thế hiện đại rùi mà cần chi cách cân đó cho vô cái cân xe là xong 

\(=2\left(\frac{2}{2.4}+\frac{2}{4.6}+...+\frac{2}{26.28}\right)=2\left(\frac{1}{2}-\frac{1}{4}+\frac{1}{4}-\frac{1}{6}+...+\frac{1}{26}-\frac{1}{28}\right)\)

\(=2\left(\frac{1}{2}-\frac{1}{28}\right)\)

1.What would you like for dinner ?

2. How much is a cake ?

3. How do you feel ?

4. How often do you go fishing ?

5.. Do you often play badminton in the fall . 6. What is the weather like in the spring ?

7. What are they doing at the moment ?

8. What do you play ?

9. What does she do in her free time ?

10. How much rice does he want ?

I like bread,milk and omelet for breakfast

Hello, every body. I’m Hoai from class 6A. If I become the president of the 3Rs Club, I will tell you about how to make my school green. Firstly, I will talk to my friends about putting a recycling bin in every classroom. We prevent lazy students from throwing garbage everywhere. Then we can reuse or sell the things we have in these bins. If we get a lot of money from selling these things, we will buy energy-saving lights for every class.

We should .clean.. the enviroment. We should..scavenging. trash and .throw.. it in the trash can. We .shouldn't..throw away empty cans and bottles . We should collect them and ..recycle. them. We shouldn't .waste.. electricity and water, too

1the area at midnight last night was searching

2.the swimming pool is being leaned by them just now