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## 计算几何基础

### 单位向量 $(Unit\ vector)$

​ 对于任意向量 $\vec a$ ，不论方向如何，若其大小为单位长度，则称其为 $\vec a$ 方向上的单位向量 $(Unit\ vector)$ 。单位向量通常被记为 $\vec u$ 。

UVA - 11235

# Area

For a given polygon g, computes the area of the polygon.

g is represented by a sequence of points $p_1$, $p_2$,…, $p_n$ where line segments connecting pi and pi+1 (1 ≤ in−1) are sides of g. The line segment connecting pn and p1 is also a side of the polygon.