第1題
如下圖,大圓的半徑為5,小圓的半徑為2,\(A\)在大圓的圓周上且為小圓的圓心,\( \overline{BC} \)為大圓的弦且與小圓相切。若\( \overline{AB}=8 \),則\( \overline{AC}= \)?
[提示]
\( \displaystyle \frac{\overline{AC}}{\sin B}=2R\)
第8題
已知長方體\(ABCD-EFGH\)如下圖所示,其中\(\overline{AB}=\overline{BF}=\sqrt{3}\)且\( \overline{AD}=1 \)。已知\( \overline{FH} \)上有一點\(P\),求\( \overline{BP}+\overline{PG} \)的最小值。
[解答]
定坐標\(F\left( 0,0,0 \right),H\left( 1,\sqrt{3},0 \right),B\left( 0,0,\sqrt{3} \right),G\left( 1,0,0 \right),P\left( t,\sqrt{3}t,0 \right)\)
\(\begin{align}
& \overline{BP}+\overline{PG} \\
& =\sqrt{{{t}^{2}}+3{{t}^{2}}+3}+\sqrt{{{\left( t-1 \right)}^{2}}+3{{t}^{2}}} \\
& =\sqrt{4{{t}^{2}}+3}+\sqrt{4{{t}^{2}}-2t+1} \\
& =\sqrt{{{\left( t+\frac{3}{2} \right)}^{2}}+{{\left( \sqrt{3}t-\frac{\sqrt{3}}{2} \right)}^{2}}}+\sqrt{{{\left( t-1 \right)}^{2}}+{{\left( \sqrt{3}t \right)}^{2}}} \\
\end{align}\)
視為\(y=\sqrt{3}x\)上一點\(\left( t,\sqrt{3}t \right)\)到\(\left( -\frac{3}{2},\frac{\sqrt{3}}{2} \right)\)和\(\left( 1,0 \right)\)的距離和之最小值