K
khodatten
![](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 co AB=AC=a, va goc BAC=alpha,voi alpha nhỏ hon 90 do va lon hon 0 do, 1,tinh BC the a va alpha.2,Goi I la tam duong tron noi tiep tam giac ABC, cmR:r=asinalpha/2(1+sinalpha/2)