Cho lăng trụ đứng
, đáy
là tam giác vuông tại
. Gọi
là trọng tâm của tam giác
và
là trung điểm cạnh
là góc giữa đường thẳng
và mặt phẳng
. Tính ![img10](https://cungthi.online/upload/questionbank3_20201220/eduquestion/2019241095575759400/obj2019241095575759400440462_images/obj2019241095575759400440462_img10.png)
![img1](https://cungthi.online/upload/questionbank3_20201220/eduquestion/2019241095575759400/obj2019241095575759400440462_images/obj2019241095575759400440462_img1.png)
![img2](https://cungthi.online/upload/questionbank3_20201220/eduquestion/2019241095575759400/obj2019241095575759400440462_images/obj2019241095575759400440462_img2.png)
![img3](https://cungthi.online/upload/questionbank3_20201220/eduquestion/2019241095575759400/obj2019241095575759400440462_images/obj2019241095575759400440462_img3.png)
![img4](https://cungthi.online/upload/questionbank3_20201220/eduquestion/2019241095575759400/obj2019241095575759400440462_images/obj2019241095575759400440462_img4.png)
![img5](https://cungthi.online/upload/questionbank3_20201220/eduquestion/2019241095575759400/obj2019241095575759400440462_images/obj2019241095575759400440462_img5.png)
![img6](https://cungthi.online/upload/questionbank3_20201220/eduquestion/2019241095575759400/obj2019241095575759400440462_images/obj2019241095575759400440462_img6.png)
![img7](https://cungthi.online/upload/questionbank3_20201220/eduquestion/2019241095575759400/obj2019241095575759400440462_images/obj2019241095575759400440462_img7.png)
![img8](https://cungthi.online/upload/questionbank3_20201220/eduquestion/2019241095575759400/obj2019241095575759400440462_images/obj2019241095575759400440462_img8.png)
![img9](https://cungthi.online/upload/questionbank3_20201220/eduquestion/2019241095575759400/obj2019241095575759400440462_images/obj2019241095575759400440462_img9.png)
![img10](https://cungthi.online/upload/questionbank3_20201220/eduquestion/2019241095575759400/obj2019241095575759400440462_images/obj2019241095575759400440462_img10.png)
A.
B.
C.
D.
Đáp án và lời giải
Đáp án:A
Lời giải:
Phân tích: Gọi
là trung điểm của
Ta có
Mà
Vậy
Đáp án đúng là A