Cac bat dang thuc hay (topic chưa sửa)

Cac ban giai cac ba`i sau, neu lo`i giai hay, khac sach va khac to' (to se kiem tra), moi loi giai treo giai 5k (moi ba`i chi co ` loi gai vip nhat duco tra tien):
1) a,b,c >0; cmr: a/b+b/c+c/a \ge 3 +(a-c)^2/(ab+bc+ca)
2)a,b,c \ge 0. cmr: \sum{\frac{(a^2(b+c)^2}{b^2+c^2}}\ge 2\sum{ab}
3) tam giac ABC. Cmr: \sum{\frac{r_ar_b}{m_am_b}}\ge 3