計算 D (2)
已知一銳角三角形\(\triangle ABC\)之邊長分別為\(a,b,c\)。令\(r\)為\(\triangle ABC\)內切圓之半徑，\(R\)為\(\triangle ABC\)外接圓之半徑，
試證：(1)\(\displaystyle r=4Rsin\frac{A}{2}sin\frac{B}{2}sin\frac{C}{2}\)
(2)\(\displaystyle \frac{abc}{\sqrt{2(a^2+b^2)(b^2+c^2)(c^2+a^2)}}\ge\frac{r}{2R}\)
[解答]
