三角形三條邊都知道了，形狀確定了，面積肯定是能算出來的。而且隱隱約約覺得有那麼個具對稱美的公式。但怎麼也想不起來了。也許重來就沒有親手推導過。是的，“美”是必須的，正如英國數學家Hardy所說。

The mathematician’s patterns, like the painter’s or the poet’s, must be beautiful; the ideas, like the colours or the words, must fit together in a harmonious way. Beauty is the first test: there is no permanent place in the world for ugly mathematics.

G. H. Hardy

那就不妨補補課。忙乎一陣子，得到

(1/4)√((b + c + a)(b + c - a)(a + c - b)(a + b - c))

感覺不錯，夠漂亮的了。找谷歌對對答案，才發現美無止境啊。

√(S(S - a)(S - b)(S - c))