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Coordinate geometry

 Angle between two lines in space Angle between two planes in space Angle between a line and a plane The angle t between two lines is the angle between two direction vectors of the lines. Take line a with direction vector u and line b with direction vector v. That is: u.v= ||u||.||v||.cos(t) and then: The angle between two planes is given by the angle between the normal vectors, n1 and n2, that is: Let's name t to the angle which line makes with plane, v the direction vector of line and n the normal vector of the plane. Then:

Choose the angle between the lines $\fs2s=\{\begin{eqnarray}-x-8y+6z&=&-4\\4x-8y+z&=&4\end{eqnarray}\\$ and $\fs2r=\{\begin{eqnarray}-2x-9y+9z&=&8\\4x+9y&=&8\end{eqnarray}\\$

 73.17 71.59 69.5 66.5

The angle is given in degrees