Tính tổng
A.
![img1](https://cungthi.online/upload/questionbank3/eduquestion/20191005616419464681/obj20191005616419464681925631_images/obj20191005616419464681925631_img1.png)
B.
![img1](https://cungthi.online/upload/questionbank3/eduquestion/20191005616419464681/obj20191005616419464681783126_images/obj20191005616419464681783126_img1.png)
C.
![img1](https://cungthi.online/upload/questionbank3/eduquestion/20191005616419464681/obj20191005616419464681640621_images/obj20191005616419464681640621_img1.png)
D.
![img1](https://cungthi.online/upload/questionbank3/eduquestion/20191005616419464681/obj20191005616419464681643556_images/obj20191005616419464681643556_img1.png)
Đáp án và lời giải
Đáp án:C
Lời giải:Phân tích:
Đặt
Xét
Viết ngược lại biểu thức của
ta được
Cộng
và
vế theo vế và kết hợp với công thức
ta có
Suy ra
Vậy đáp án đúng là C.
![img1](https://cungthi.online/upload/questionbank3/eduquestion/20191005616419464681/obj20191005616419464681946962_images/obj20191005616419464681946962_img1.png)
![img2](https://cungthi.online/upload/questionbank3/eduquestion/20191005616419464681/obj20191005616419464681946962_images/obj20191005616419464681946962_img2.png)
![img3](https://cungthi.online/upload/questionbank3/eduquestion/20191005616419464681/obj20191005616419464681946962_images/obj20191005616419464681946962_img3.png)
![img4](https://cungthi.online/upload/questionbank3/eduquestion/20191005616419464681/obj20191005616419464681946962_images/obj20191005616419464681946962_img4.png)
![img5](https://cungthi.online/upload/questionbank3/eduquestion/20191005616419464681/obj20191005616419464681946962_images/obj20191005616419464681946962_img5.png)
![img6](https://cungthi.online/upload/questionbank3/eduquestion/20191005616419464681/obj20191005616419464681946962_images/obj20191005616419464681946962_img6.png)
![img7](https://cungthi.online/upload/questionbank3/eduquestion/20191005616419464681/obj20191005616419464681946962_images/obj20191005616419464681946962_img7.png)
![img8](https://cungthi.online/upload/questionbank3/eduquestion/20191005616419464681/obj20191005616419464681946962_images/obj20191005616419464681946962_img8.png)
![img9](https://cungthi.online/upload/questionbank3/eduquestion/20191005616419464681/obj20191005616419464681946962_images/obj20191005616419464681946962_img9.png)
![img10](https://cungthi.online/upload/questionbank3/eduquestion/20191005616419464681/obj20191005616419464681946962_images/obj20191005616419464681946962_img10.png)
![img11](https://cungthi.online/upload/questionbank3/eduquestion/20191005616419464681/obj20191005616419464681946962_images/obj20191005616419464681946962_img11.png)
![img12](https://cungthi.online/upload/questionbank3/eduquestion/20191005616419464681/obj20191005616419464681946962_images/obj20191005616419464681946962_img12.png)