106文華高中代理

\(\angle P'OP" = 2\theta\)

\(\displaystyle \cos2\theta=\frac{-7}{25}, \overline{OP'} = \overline{OP''} = \overline{OP}=10\Rightarrow\) 用餘弦定理可得 \(\overline{P'P''}\)