填充題,第 15 題,
以 \(O\) 為原點之坐標平面,若 \(\displaystyle \overrightarrow{OP}=\left(3\sin\alpha+\cos\beta, \sin\alpha+3\cos\beta\right)\),
\(\displaystyle 0\le\alpha\le\frac{\pi}{6},0\le\beta\le\frac{\pi}{3}\),則 \(\overrightarrow{OP}\) 之一切 \(P\) 點所成區域的面積為何?
解答:
\(\displaystyle \overrightarrow{OP}=\sin\alpha\cdot (3,1)+\cos\beta\cdot (1,3)\),
因為 \(\displaystyle 0\le\alpha\le\frac{\pi}{6},\;0\le\beta\le\frac{\pi}{3}\),
所以 \(\displaystyle 0\le\sin\alpha\le\frac{1}{2},\;\frac{1}{2}\le\cos\beta\le 1\).
因此,\(P\) 點所成區域的面積\(\displaystyle =\left(\frac{1}{2}-0\right)\left(1-\frac{1}{2}\right)\cdot\{(3,1)\mbox{ 與 } (1,3) \mbox{所形成的平行四邊形面積}\}\)
\(\displaystyle \qquad\qquad\qquad\qquad\qquad=\frac{1}{4}|\left| {\begin{array}{*{20}{c}}
3 & 1 \\
1 & 3 \\
\end{array}} \right||=2.\)