Question

Даны точки a(5; 2; -1),b(1; -1; 0),c(3; -1; 0).найти координаты векторного произведения [bc-2ca,cb].

Answer 1

ответ: {0; -4; -12}

$A(5;2;-1) \quad B(1;-1;0) \quad C(3;-1;0)\\\\\overline {BC} = \{3 - 1; -1 - (-1); 0-0 \} = \{2;0; 0 \}\\\\\overline {CA} = \{5-3;2-(-1); -1 - 0 \} = \{2;3;-1 \}\\ \\ \overline a = \overline {BC} - 2\cdot \overline{CA} = \{2-2\cdot 2; 0-2\cdot 3; 0 - 2\cdot(-1)\} = \{-2; -6; 2\} \\ \\ \overline b =\overline {CB} = -\overline {BC} = \{-2; 0; 0 \}$

Найдём векторное произведение

$\overline a \times \overline b = \left|\begin{array}{ccc}\overline i &\overline j&\overline k\\-2&-6&2\\-2&0&0\end{array}\right| = \overline i \left|\begin{array}{rc} -6 & 2 \\ 0 & 0\end{array}\right| - \overline j \left|\begin{array}{rc} -2 & 2 \\ -2 & 0 \end{array}\right|+\overline k \left|\begin{array}{rc} -2 & -6 \\ -2 & 0\end{array}\right| = \\ \\ \\ = 0\,\overline i -4\overline j - 12\overline k = \{0;-4; -12\}$