Cho tứ diện
có
,
,
đôi một vuông góc và
,
. Tính góc giữa hai mặt phẳng
và
.
![img1](https://cungthi.online/upload/questionbank3/eduquestion/2019141476921859821/obj2019141476921859821540883_images/obj2019141476921859821540883_img1.png)
![img2](https://cungthi.online/upload/questionbank3/eduquestion/2019141476921859821/obj2019141476921859821540883_images/obj2019141476921859821540883_img2.png)
![img3](https://cungthi.online/upload/questionbank3/eduquestion/2019141476921859821/obj2019141476921859821540883_images/obj2019141476921859821540883_img3.png)
![img4](https://cungthi.online/upload/questionbank3/eduquestion/2019141476921859821/obj2019141476921859821540883_images/obj2019141476921859821540883_img4.png)
![img5](https://cungthi.online/upload/questionbank3/eduquestion/2019141476921859821/obj2019141476921859821540883_images/obj2019141476921859821540883_img5.png)
![img6](https://cungthi.online/upload/questionbank3/eduquestion/2019141476921859821/obj2019141476921859821540883_images/obj2019141476921859821540883_img6.png)
![img7](https://cungthi.online/upload/questionbank3/eduquestion/2019141476921859821/obj2019141476921859821540883_images/obj2019141476921859821540883_img7.png)
![img8](https://cungthi.online/upload/questionbank3/eduquestion/2019141476921859821/obj2019141476921859821540883_images/obj2019141476921859821540883_img8.png)
A.
B.
C.
D.
Đáp án và lời giải
Đáp án:D
Lời giải:
Phân tích: Gọi
là trung điểm của
. Mà
nên
. Ta có:
. Ta có:
. Xét tam giác
vuông tại
có
. Vậy
.
Vậy đáp án đúng là D.