行列式的應用
行列式的應用(Applications of Determinant)
國立臺南第一高級中學林倉億老師
連結:行列式的性質
以下介紹行列式在高中數學中主要的應用:
- 表示平面上三角形的面積
\(\vec{OA}=(a_1,b_1)\)、\(\vec{OB}=(a_2,b_2)\),則 \(\Delta OAB\) 面積 \(= \frac{1}{2}\left| {\left| { \begin{array}{*{20}{c}} {{a_{ 1}}}&{{b_{ 1}}}\\ {{a_{ 2}}}&{{b_{ 2}}} \end{array} } \right|} \right|\)
(\(\frac{1}{2}\) 乘以 \(\left| { \begin{array}{*{20}{c}} {{a_{ 1}}}&{{b_{ 1}}}\\ {{a_{ 2}}}&{{b_{ 2}}} \end{array} } \right|\) 的絕對值)。
【證明】:
由三角形面積公式 \(\Delta OAB = \frac{1}{2}\sqrt {{{\left| {\vec{OA}} \right|}^2}{{\left| {\vec{OB}} \right|}^2} – {{\left( {\vec{OA} \cdot\vec{OB}} \right)}^2}}\)
可得
\(\begin{array}{ll}\Delta OAB &= \frac{1}{2}\sqrt {({a_{ 1}}^2 + {b_{ 1}}^2)({a_{ 2}}^2 + {b_{ 2}}^2) – {{\left( {{a_{ 1}}{a_{ 2}} + {b_{ 1}}{b_{ 2}}} \right)}^2}} \\&= \frac{1}{2}\sqrt {{a_{ 1}}^2{b_{ 2}}^2 + {a_{ 2}}^2{b_{ 1}}^2 – 2{a_{ 1}}{a_{ 2}}{b_{ 1}}{b_{ 2}}} = \frac{1}{2}\sqrt {{{\left( {{a_{ 1}}{b_{ 2}} – {a_{ 2}}{b_{ 1}}} \right)}^2}} \\&= \frac{1}{2}\sqrt { {{\left| { \begin{array}{*{20}{c}} {{a_{ 1}}}&{{b_{ 1}}}\\ {{a_{ 2}}}&{{b_{ 2}}} \end{array} } \right|}^2}} = \frac{1}{2}\left| {\left| { \begin{array}{*{20}{c}} {{a_{ 1}}}&{{b_{ 1}}}\\ {{a_{ 2}}}&{{b_{ 2}}} \end{array} } \right|} \right|\end{array}\) - 表示空間中兩不平行向量的外積
\(\vec{OA}=(a_1,b_1,c_1)\),\(\vec{OB}=(a_2,b_2,c_2)\),則 \(\vec{OA}\) 與 \(\vec{OB}\) 的外積
可記作 \(\vec{OA}\times\vec{OB}= ( \left| { \begin{array}{*{20}{c}} {{b_{ 1}}}&{{c_{ 1}}}\\ {{b_{ 2}}}&{{c_{ 2}}} \end{array} } \right| , \left| { \begin{array}{*{20}{c}} {{c_{ 1}}}&{{a_{ 1}}}\\ {{c_{ 2}}}&{{a_{ 2}}} \end{array} } \right| , \left| { \begin{array}{*{20}{c}} {{a_{ 1}}}&{{b_{ 1}}}\\ {{a_{ 2}}}&{{b_{ 2}}} \end{array} } \right| )\)