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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 giac ABC
a/ cos^2 {A} + cos^2 {B} +cos^2 {C} \geq 3/4
b/ 2\leqcos^2 {A/2} +cos^2 {B/2} + cos^2 {C/2} \leq9/4
c/ cos {A/2}.cos{B/2}.cos {C/2}\leq3.can3/8
d/ 1/cos A +1/cos B + 1/cos C \geq 6 voi A, B, C la 3 goc nhon
a/ cos^2 {A} + cos^2 {B} +cos^2 {C} \geq 3/4
b/ 2\leqcos^2 {A/2} +cos^2 {B/2} + cos^2 {C/2} \leq9/4
c/ cos {A/2}.cos{B/2}.cos {C/2}\leq3.can3/8
d/ 1/cos A +1/cos B + 1/cos C \geq 6 voi A, B, C la 3 goc nhon