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![](https://blog.hocmai.vn/wp-content/uploads/2017/07/hot.gif)
![](https://blog.hocmai.vn/wp-content/uploads/2017/07/hot.gif)
Cho tam giác ABC ,CM rằng:
[tex]a)cosA +cosB + cosC =4.sin \frac{A}/{2}. sin \frac{B}/{2} .sin \frac{C}/{2} \\ b) sin2A +sin2B +sin2C =4sinA sinB sinC \\ c) cos2A +cos2B + cos2C =-1-4cosA cosB cosC[/tex]
[tex]a)cosA +cosB + cosC =4.sin \frac{A}/{2}. sin \frac{B}/{2} .sin \frac{C}/{2} \\ b) sin2A +sin2B +sin2C =4sinA sinB sinC \\ c) cos2A +cos2B + cos2C =-1-4cosA cosB cosC[/tex]