Một bóng đèn nhỏ S đặt cách tường khoảng= SH= 1m. Tại trung điểm M người t đặt một miếng bìa hình tròn bán kính 30 cm song song với tường. Bán kính bóng đen trên tường là bao nhiêu?

2 câu trả lời

Giải thích các bước giải:

Xem hình  ảnh đình kèm

Bức tường là đoạn EK

Tấm bìa hình tròn đoạn AB

Do M là trung điểm của đoạn thẳng SH                (1)

⇒ SM=MH=1/2 SH

Mà SH=1m

⇒ SM = MH = 0,5m

Vì AB//EK

⇒ AM//EH                                                                 (2)

Xét Δ SEH, ta có: 

SM/MH = SA/AE   ( Theo định lý Talet)

Mà SM = MH = 0,5m

Ta có SM/MH = SA/AE

⇒ 0,5m/ 0,5 = SA/AE

⇒ 1m = SA = AE

⇒ A là trung điểm của đoạn thẳng SE                           (3)

Từ (1) , (2) , (3) ta có:

+ M là trung điểm của đoạn thẳng SH

+ AM//EH

+ A là trung điểm của đoạn thẳng SE

AM là đường trung bình của Δ SEH

Vậy bán kính bóng đen trên tường là 60cm

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Chúc bạn học tốt nhé

Đáp án.

nguồn sáng rộng có bán kính r = 2cm.

Giải thích các bước giải:

SH = 1m = 100cm

SM = 50 cm; R = 10 cm

Rnửa tối = 4 cm; Rtối = 18 cm

Tìm r.

Bài giải:

Ta có hình vẽ

HP = 18 cm và PO = 4cm

IM = HH’= R = 10 cm.

Mà PH’ = HP –HH’ = 18 - 10 = 8 cm

Ta có: PH’ = AA’ (vì ∆ AA’I = ∆PH’I )

AA’ = SA’ – SA = MI – SA = R – r

Vậy 8 = R – r = 10 – r

Suy ra r = 2 cm.

Câu hỏi trong lớp Xem thêm

I. Circle the correct one. “Too or Enough” 1) This car is too/enough old. We cannot go anywhere. 2) This book is too/enough thick for me to read. 3) That dress is cheap too/enough to buy. 4) This question is too/enough complicated to answer. 5) Everybody is too/enough excited about the holiday to focus on their work. 6) I can join that club. I’m young too/enough. 7) She can’t come with us to the bar. She’s too/enough young. 8) We have too/enough eggs to make a cake. 9) Do we have too/enough time to catch the bus? 10)This boy is too/enough noisy. I can’t study! 11)Mary’s room is too/enough dirty. Her mom is angry. 12)Tom is tall too/enough to join a basketball team. 13)This bag is too/enough heavy. I can’t carry it. II. Complete with “too” or “enough”. 1- She is clever ........................ to pass the test. 2- The skirt is ........................ long for you. 3- Kate is .......................young to drive a car. 4- There is ......................... meat in the fridge for the whole week. 5- I didn’t work hard ........................ to pass the exam. 6- We can’t buy the car because it is .......................expensive. 7- He can’t sleep because he drinks .......................much coffee. 8- Are you tall ...................... to reach that shelf? 9- They often take the bus to school because it’s ................... far to walk. 10- She isn’t fast ...................... to win the race. 11- There’s .......................food for everyone to eat. 12- She isn’t speaking clearly ................for me to understand. III. Complete the sentences with “too – enough” by using the words given. 1) I don’t want to eat that soup. It is _______________________. ( cold ) 2) She can’t come to party with us. She isn’t _______________________. ( old ) 3) They need much money. That car is _______________________ to buy. ( expensive ) 4) We don’t need to worry about the food. It is _______________________. (fresh ) 5) I missed the bus because I got up _______________________. ( late ) 6) I cannot buy that dress. It isn’t _______________________ for me to buy. ( cheap ) 7) I cannot climb that mountain. It is _______________________. ( high ) 8) You should help your sister. She is _______________________ to eat herself. (young) 9) It is _______________________ to see the way. ( foggy ) 10) He is _______________________ to win the race. ( fast ) 11) He is _______________________ to be the headmaster. ( qualified ) 12) Don’t want any help from him. He is _____________________ to help anyone. (rude) 13) The movie was _______________________. I couldn’t watch it. ( boring ) 14) Ben is _______________________ to solve the problems. He never studies. ( lazy ) 15) The questions were _______________________ to solve. ( difficult ) 16) Jeremy is _______________________ to talk people. ( shy ) 17) Ashley is _______________________ to pass the test. ( hardworking ) 18) I can’t wear this t-shirt. It is _______________________. ( small ) Giúp mk vs, tuy ns dài nhưg rấc cần sự giúp đỡ! Mk cần 2 bn lm ạ, bn nào có sức thì làm 2 bài, còn bn còn lại lm 1 bài a

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