If the angle between lines $AB$ and $CD$ is $\theta$,then the projection of line segment $AB$ on $CD$ is = ..........

The direction ratios of the line joining the points $(4, 3, -5)$ and $(-2, 1, -8)$ are

If the direction cosines of a line are $\left( \frac{1}{c}, \frac{1}{c}, \frac{1}{c} \right)$,then

The angle between the lines whose direction ratios satisfy the equations $l+m+n=0$ and $l^2=m^2+n^2$ is

Suppose $(l_1, m_1, n_1)$ and $(l_2, m_2, n_2)$ are the direction cosines of two lines and $\theta$ is the angle between them,where $\cos \theta = \pm(l_1 l_2 + m_1 m_2 + n_1 n_2)$. Let $A=(1, -2, 3)$,$B=(3, 1, -3)$,and $C=(-3, 1, 3)$ be the vertices of $\triangle ABC$. Then,$\cos A =$

Find the direction cosines of the sides of the triangle whose vertices are $(3,5,-4), (-1,1,2)$ and $(-5,-5,-2)$.