I cant find the angle between the vertices c (1,1,1) a (1,-4,2) b (-5,2,7) Can you help me?
I set:
\(\small{ \begin{array}{lcr} a_x &=& 1\\ a_y&=&-4\\ a_z&=&2 \end{array} \qquad \begin{array}{lcr} b_x &=& -5\\ b_y&=&2\\ b_z&=&7 \end{array} \qquad \begin{array}{lcr} c_x &=& 1\\ c_y &=& 1\\ c_z &=& 1\\ \end{array} }\)
Differences of the vectors:
\(\small{ \begin{array}{lcr} \vec{b}-\vec{a} &=& \bigl(\begin{smallmatrix} b_x-a_x\\b_y-a_y\\b_z-a_z \end{smallmatrix}\bigr) = \bigl(\begin{smallmatrix} -6\\6\\5 \end{smallmatrix}\bigr)\\ \vec{a}-\vec{b} &=& -\bigl(\begin{smallmatrix} -6\\6\\5 \end{smallmatrix}\bigr) =\bigl(\begin{smallmatrix} 6\\-6\\-5 \end{smallmatrix}\bigr) \\ \end{array} \qquad \begin{array}{lcr} \vec{c}-\vec{b} &=& \bigl(\begin{smallmatrix} c_x-b_x\\c_y-b_y\\c_z-b_z \end{smallmatrix}\bigr) = \bigl(\begin{smallmatrix} 6\\-1\\-6 \end{smallmatrix}\bigr)\\ \vec{b}-\vec{c} &=& -\bigl(\begin{smallmatrix} 6\\-1\\-6 \end{smallmatrix}\bigr) =\bigl(\begin{smallmatrix} -6\\1\\6 \end{smallmatrix}\bigr) \\ \end{array} \qquad \begin{array}{lcr} \vec{a}-\vec{c} &=& \bigl(\begin{smallmatrix} a_x-c_x\\a_y-c_y\\a_z-c_z \end{smallmatrix}\bigr) = \bigl(\begin{smallmatrix} 0\\-5\\1 \end{smallmatrix}\bigr)\\ \vec{c}-\vec{a} &=& -\bigl(\begin{smallmatrix} 0\\-5\\1 \end{smallmatrix}\bigr) =\bigl(\begin{smallmatrix} 0\\5\\-1 \end{smallmatrix}\bigr) \\ \end{array} }\)
In the plane of the triangle (ABC) the angles are:
\(\small{ \begin{array}{rcl} \tan{(A)} &=& \frac{ \vert (\vec{c}-\vec{a}) \times (\vec{b}-\vec{a}) \vert} {(\vec{c}-\vec{a})\cdot (\vec{b}-\vec{a}) } = \frac{ \left| \bigl(\begin{smallmatrix} 0\\5\\-1 \end{smallmatrix}\bigr) \times \bigl(\begin{smallmatrix} -6\\6\\5 \end{smallmatrix}\bigr) \right|} {\bigl(\begin{smallmatrix} 0\\5\\-1 \end{smallmatrix}\bigr)\cdot \bigl(\begin{smallmatrix} -6\\6\\5 \end{smallmatrix}\bigr) } = \frac{ 43.5545634808 } {25 } \qquad (I.)\\ A &=& \arctan{(1.74218253923)}\\ A &=& 60.1444921468^\circ \\ \tan{(B)} &=& \frac{ \vert (\vec{a}-\vec{b}) \times (\vec{c}-\vec{b}) \vert} {(\vec{a}-\vec{b})\cdot (\vec{c}-\vec{b}) } = \frac{ \left| \bigl(\begin{smallmatrix} 6\\-6\\-5 \end{smallmatrix}\bigr) \times \bigl(\begin{smallmatrix} 6\\-1\\-6 \end{smallmatrix}\bigr) \right|} {\bigl(\begin{smallmatrix} 6\\-6\\-5 \end{smallmatrix}\bigr)\cdot \bigl(\begin{smallmatrix} 6\\-1\\-6 \end{smallmatrix}\bigr) } = \frac{ 43.5545634808 } {72 } \qquad (I.)\\ B &=& \arctan{(0.60492449279)}\\ B &=& 31.1707710617^\circ \\ \tan{(C)} &=& \frac{ \vert (\vec{b}-\vec{c}) \times (\vec{a}-\vec{c}) \vert} {(\vec{b}-\vec{c})\cdot (\vec{a}-\vec{c}) } = \frac{ \left| \bigl(\begin{smallmatrix} -6\\1\\6 \end{smallmatrix}\bigr) \times \bigl(\begin{smallmatrix} 0\\-5\\1 \end{smallmatrix}\bigr) \right|} {\bigl(\begin{smallmatrix} -6\\1\\6 \end{smallmatrix}\bigr)\cdot \bigl(\begin{smallmatrix} 0\\-5\\1 \end{smallmatrix}\bigr)} = \frac{ 43.5545634808 } {1 } \qquad (I.)\\ C &=& \arctan{(43.5545634808)}\\ C &=& 88.6847367915^\circ \\ \end{array} }\)
\(A+B+C = 60.1444921468^\circ + 31.1707710617^\circ + 88.6847367915^\circ = 180^\circ \)
