向量：正射影

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公式：
1. $\overrightarrow{AD}=\left( r_{1}\cos \Delta \theta \right)\times \left( \frac{1}{r_{2}}\times \overrightarrow{AC} \right)$ 註： $\overrightarrow{AB}$ 在 $\overrightarrow{AC}$ 方向上的分量乘以 $\overrightarrow{AC}$ 方向的單位向量
2. $\overrightarrow{AD}=\overrightarrow{AB}\cdot \overrightarrow{AC}\times \frac{\overrightarrow{AC}}{\left| \overrightarrow{AC} \right|^{2}}$
3. $\overrightarrow{AD}=\left( x_{1}x_{2}+y_{1}y_{2} \right)\times \frac{1}{\left( x_{2}^{2}+y_{2}^{2} \right)}\times \overrightarrow{AC}$

$\overrightarrow{AB}=\left( r_{1}\cos \theta _{1},r_{1}\sin \theta _{1} \right)=\left( \Delta x_{1},\Delta y_{1} \right)$
$\overrightarrow{AC}=\left( r_{2}\cos \theta _{2},r_{2}\sin \theta _{2} \right)=\left( \Delta x_{2},\Delta y_{2} \right)$
$\theta _{2}\,$ 方向角之單位向量 $\left( \cos \theta _{2},\sin \theta _{2} \right)=\left( \frac{r_{2}}{r_{2}}\cos \theta _{2},\frac{r_{2}}{r_{2}}\sin \theta _{2} \right)$
$=\frac{1}{r_{2}}\left( r_{2}\cos \theta _{2},r_{2}\sin \theta _{2} \right)=\frac{1}{r_{2}}\times \overrightarrow{AC}$
$\overrightarrow{AD}=\left( r_{1}\cos \Delta \theta \right)\times \left( \cos \theta _{2},\sin \theta _{2} \right)$
$=\left( r_{1}\cos \Delta \theta \right)\times \left( \frac{1}{r_{2}}\times \overrightarrow{AC} \right)\#$
$=r_{1}r_{2}\cos \Delta \theta \times \frac{1}{r_{2}^{2}}\times \overrightarrow{AC}$
$=\left( x_{1}x_{2}+y_{1}y_{2} \right)\times \frac{1}{\left( x_{2}^{2}+y_{2}^{2} \right)}\times \overrightarrow{AC}\#$